feat: files pane like VS Code: tree, tabs, go to file

- Explorer tree with inline expand/collapse and keyboard navigation, git
  colours, change dots, dimmed gitignored entries (fs-list now flags them),
  file-type badges, resizable explorer.
- Editor tabs: preview tab semantics, instant switching with scroll kept,
  in-place reload when Claude changes an open file.
- Go to file: devdesk attach --fs-find (git ls-files or capped find) and a
  fuzzy-ranked palette (CommandPalette gained custom search/async lists).
- Per-session memory of folders and tabs; empty state with recents.

Closes #37

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/styles.css

@@ -700,19 +700,21 @@ html[data-theme="light"] .lt-notif-dot { box-shadow: none; }
   .dd-inbox-panel { position: fixed; top: calc(var(--header-height) + 4px); right: 8px; left: 8px; width: auto; }
 }
 
-/* ---- files pane (devdesk#19) -------------------------------------------------------- */
+/* ---- files pane (devdesk#19, #37): explorer + tabs + editor ---------------------- */
 .dd-files {
   position: relative;
   display: flex;
   flex-direction: column;
-  flex: 0 0 var(--dd-files-w, 46%);
+  flex: 0 0 var(--dd-files-w, 52%);
   min-width: 0;
   margin: 0;
   background: var(--bg-terminal);
+  --dd-explorer-w: 240px;
 }
 .dd-files[hidden] { display: none; }
 body.dd-resizing { cursor: col-resize; user-select: none; }
-body.dd-resizing .dd-terminal { pointer-events: none; }
+body.dd-resizing .dd-terminal,
+body.dd-resizing .dd-md-frame { pointer-events: none; }
 .dd-files-resizer {
   position: absolute;
   top: 0;
@@ -727,72 +729,137 @@ body.dd-resizing .dd-terminal { pointer-events: none; }
 body.dd-resizing .dd-files-resizer { background: linear-gradient(90deg, transparent 40%, var(--accent-cyan) 40%, var(--accent-cyan) 60%, transparent 60%); outline: none; }
 #files-toggle[aria-expanded="true"] { color: var(--accent-orange); border-color: var(--accent-orange-border); }
 
-.dd-files-head { display: flex; align-items: flex-end; justify-content: space-between; gap: 8px; padding: 2px 6px 0 6px; border-bottom: 1px solid var(--border-dim); }
-.dd-files-tabs { display: flex; }
-.dd-files-tabs button {
-  padding: 7px 10px 6px;
+/* head: explorer toggle | tabs | actions */
+.dd-files-head { display: flex; align-items: stretch; gap: 2px; min-height: 34px; padding: 0 4px; border-bottom: 1px solid var(--border-dim); background: var(--bg-secondary); }
+.dd-files-head .dd-icon-btn { align-self: center; }
+.dd-files-actions { display: flex; align-items: center; gap: 2px; margin-left: auto; }
+.dd-tabs { display: flex; flex: 1; min-width: 0; overflow-x: auto; scrollbar-width: thin; }
+.dd-tab {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  max-width: 220px;
+  padding: 0 6px 0 10px;
+  border-right: 1px solid var(--border-dim);
+  border-top: 2px solid transparent;
+  color: var(--text-muted);
+  font-family: var(--font-mono);
+  font-size: 0.74rem;
+  white-space: nowrap;
+  cursor: pointer;
+  user-select: none;
+}
+.dd-tab:hover { color: var(--text-primary); background: var(--bg-tertiary); }
+.dd-tab.is-active { color: var(--text-primary); background: var(--bg-terminal); border-top-color: var(--accent-orange); }
+.dd-tab.is-preview .dd-tab-name { font-style: italic; }
+.dd-tab-name { overflow: hidden; text-overflow: ellipsis; }
+.dd-tab-tag { padding: 0 4px; border: 1px solid var(--border-color); color: var(--text-muted); font-size: 0.6rem; }
+.dd-tab-close { width: 18px; height: 18px; padding: 0; border: 0; background: none; color: inherit; opacity: 0; font-size: 0.9rem; line-height: 1; cursor: pointer; }
+.dd-tab:hover .dd-tab-close,
+.dd-tab.is-active .dd-tab-close,
+.dd-tab-close:focus-visible { opacity: 0.7; }
+.dd-tab-close:hover { opacity: 1; color: var(--accent-red); }
+.dd-tab.git-amber .dd-tab-name { color: var(--accent-amber); }
+.dd-tab.git-green .dd-tab-name { color: var(--accent-green); }
+.dd-tab.git-red .dd-tab-name { color: var(--accent-red); text-decoration: line-through; }
+
+/* main: explorer column | editor */
+.dd-files-main { position: relative; display: flex; flex: 1; min-height: 0; }
+.dd-explorer { display: flex; flex-direction: column; flex: 0 0 var(--dd-explorer-w); min-width: 0; border-right: 1px solid var(--border-dim); background: var(--bg-secondary); }
+.dd-files.explorer-hidden .dd-explorer,
+.dd-files.explorer-hidden .dd-explorer-resizer,
+.dd-files.is-narrow .dd-explorer-resizer { display: none; }
+.dd-explorer-resizer { flex: 0 0 4px; margin-left: -2px; margin-right: -2px; cursor: col-resize; z-index: 2; }
+.dd-explorer-resizer:hover { background: var(--accent-cyan); }
+.dd-explorer-views { display: flex; border-bottom: 1px solid var(--border-dim); }
+.dd-explorer-views button {
+  flex: 1;
+  padding: 6px 4px 5px;
   border: 0;
   border-bottom: 2px solid transparent;
   background: none;
   color: var(--text-muted);
   font-family: var(--font-mono);
-  font-size: 0.7rem;
+  font-size: 0.64rem;
   letter-spacing: 0.1em;
   text-transform: uppercase;
   cursor: pointer;
 }
-.dd-files-tabs button:hover { color: var(--text-primary); }
-.dd-files-tabs button[aria-selected="true"] { color: var(--accent-orange); border-bottom-color: var(--accent-orange); }
-.dd-files-count:not(:empty) { margin-left: 4px; padding: 0 5px; border-radius: 8px; background: var(--accent-amber-dim); color: var(--accent-amber); font-size: 0.62rem; }
-.dd-files-actions { display: flex; gap: 2px; padding-bottom: 4px; }
+.dd-explorer-views button:hover { color: var(--text-primary); }
+.dd-explorer-views button[aria-selected="true"] { color: var(--accent-orange); border-bottom-color: var(--accent-orange); }
+.dd-files-count:not(:empty) { margin-left: 3px; padding: 0 5px; border-radius: 8px; background: var(--accent-amber-dim); color: var(--accent-amber); font-size: 0.6rem; }
+.dd-explorer-body { flex: 1; min-height: 0; overflow: auto; padding: 4px 0 12px; outline: none; }
 
-.dd-files-crumbs { display: flex; flex-wrap: wrap; align-items: center; gap: 1px; min-height: 28px; padding: 4px 10px; border-bottom: 1px solid var(--border-dim); font-family: var(--font-mono); font-size: 0.72rem; }
-.dd-crumb { padding: 1px 3px; border: 0; background: none; color: var(--accent-cyan); font: inherit; cursor: pointer; }
-.dd-crumb:hover { text-decoration: underline; }
-.dd-crumb.is-current,
-.dd-crumb.is-file { color: var(--text-primary); text-decoration: none; cursor: default; }
-.dd-crumb-sep { color: var(--text-dim); }
-
-.dd-files-body { display: flex; flex-direction: column; flex: 1; min-height: 0; overflow: auto; outline: none; }
-.dd-file-list { margin: 0; padding: 4px 0; list-style: none; }
-.dd-file {
+.dd-tree-row {
   display: flex;
   align-items: center;
-  gap: 8px;
-  width: 100%;
-  padding: 4px 10px;
-  border: 0;
-  background: none;
+  gap: 6px;
+  min-height: 24px;
+  padding-right: 8px;
   color: var(--text-primary);
   font-family: var(--font-mono);
   font-size: 0.76rem;
-  text-align: left;
+  white-space: nowrap;
   cursor: pointer;
+  user-select: none;
 }
-.dd-file:hover:not(:disabled),
-.dd-file:focus-visible { background: var(--accent-cyan-dim); outline: none; }
-.dd-file:disabled { opacity: 0.55; cursor: default; }
-.dd-file-icon { flex: 0 0 12px; color: var(--text-dim); text-align: center; }
-.dd-file.is-dir .dd-file-icon,
-.dd-file.is-dir .dd-file-name { color: var(--accent-cyan); }
-.dd-file.is-up .dd-file-name { color: var(--text-muted); }
-.dd-file-name { flex: 1; min-width: 0; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
-.dd-file-meta { color: var(--text-muted); font-size: 0.66rem; white-space: nowrap; }
-.dd-file-age { flex: 0 0 2.6em; text-align: right; }
-.dd-file-status { flex: 0 0 1.2em; font-weight: 700; text-align: center; }
+.dd-tree-row:hover { background: var(--bg-tertiary); }
+.dd-tree-row:focus-visible { outline: 1px solid var(--accent-cyan); outline-offset: -1px; }
+.dd-tree-row.is-active { background: var(--accent-cyan-dim); box-shadow: inset 2px 0 0 var(--accent-cyan); }
+.dd-tree-row.is-ignored { opacity: 0.5; }
+.dd-tree-row.is-note { color: var(--text-muted); font-size: 0.7rem; cursor: default; }
+.dd-tree-row.is-disabled { opacity: 0.5; cursor: default; }
+.dd-tree-name { flex: 1; min-width: 0; overflow: hidden; text-overflow: ellipsis; }
+.dd-tree-row.git-amber .dd-tree-name { color: var(--accent-amber); }
+.dd-tree-row.git-green .dd-tree-name { color: var(--accent-green); }
+.dd-tree-row.git-red .dd-tree-name { color: var(--accent-red); text-decoration: line-through; }
+.dd-tree-row.git-cyan .dd-tree-name { color: var(--accent-cyan); }
+.dd-tree-row.git-dirty > .dd-tree-name { color: var(--accent-amber); }
+.dd-dirty-dot { flex: 0 0 6px; height: 6px; border-radius: 50%; background: var(--accent-amber); }
+.dd-change-dir { color: var(--text-muted); font-size: 0.66rem; }
+.dd-changes-head { padding: 6px 10px; color: var(--text-muted); font-size: 0.68rem; border-bottom: 1px solid var(--border-dim); }
+
+/* file-type badges */
+.dd-ficon {
+  flex: 0 0 18px;
+  font-family: var(--font-mono);
+  font-size: 0.56rem;
+  font-weight: 700;
+  letter-spacing: -0.04em;
+  text-align: center;
+  line-height: 1;
+}
+.dd-ficon.tone-dir { color: var(--text-muted); font-size: 0.7rem; }
+.dd-ficon.tone-cyan { color: var(--accent-cyan); }
+.dd-ficon.tone-amber { color: var(--accent-amber); }
+.dd-ficon.tone-blue { color: #5b9cff; }
+.dd-ficon.tone-green { color: var(--accent-green); }
+.dd-ficon.tone-purple { color: var(--accent-purple); }
+.dd-ficon.tone-orange { color: var(--accent-orange); }
+.dd-ficon.tone-muted { color: var(--text-muted); }
+html[data-theme="light"] .dd-ficon.tone-blue { color: #1f4fbf; }
+
+.dd-file-status { flex: 0 0 1.1em; font-weight: 700; text-align: center; font-size: 0.7rem; }
 .dd-file-status.is-amber { color: var(--accent-amber); }
 .dd-file-status.is-green { color: var(--accent-green); }
 .dd-file-status.is-red { color: var(--accent-red); }
 .dd-file-status.is-cyan { color: var(--accent-cyan); }
-.dd-claude-chip { padding: 0 4px; border: 1px solid var(--accent-orange-border); color: var(--accent-orange); font-size: 0.6rem; font-weight: 700; }
-.dd-add { color: var(--accent-green); }
-.dd-del { color: var(--accent-red); }
+.dd-claude-chip { padding: 0 4px; border: 1px solid var(--accent-orange-border); color: var(--accent-orange); font-size: 0.58rem; font-weight: 700; }
+.dd-add { color: var(--accent-green); font-size: 0.66rem; }
+.dd-del { color: var(--accent-red); font-size: 0.66rem; }
 .dd-files-msg { padding: 14px; color: var(--text-muted); font-size: 0.76rem; line-height: 1.5; }
 .dd-files-msg.is-error { color: var(--accent-red); }
 
-.dd-viewer-bar { display: flex; flex-wrap: wrap; align-items: center; gap: 6px 8px; padding: 4px 8px; border-bottom: 1px solid var(--border-dim); }
-.dd-viewer-meta { flex: 1; min-width: 8em; color: var(--text-muted); font-size: 0.68rem; }
-.dd-viewer-actions { display: flex; flex-wrap: wrap; gap: 4px; }
+/* editor area */
+.dd-editor-area { position: relative; display: flex; flex: 1; min-width: 0; flex-direction: column; }
+.dd-tab-view { display: flex; flex: 1; min-height: 0; flex-direction: column; }
+.dd-tab-view[hidden] { display: none; }
+.dd-viewer-bar { display: flex; align-items: center; gap: 8px; min-height: 30px; padding: 3px 8px; border-bottom: 1px solid var(--border-dim); }
+.dd-viewer-where { flex: 1; min-width: 0; overflow: hidden; font-size: 0.7rem; text-overflow: ellipsis; white-space: nowrap; }
+.dd-viewer-path { color: var(--text-secondary); }
+.dd-viewer-meta { color: var(--text-muted); font-size: 0.66rem; transition: color 0.6s; }
+.dd-viewer-meta.is-flash { color: var(--accent-amber); }
+.dd-viewer-actions { display: flex; flex: 0 0 auto; gap: 4px; }
 .dd-viewer-btn {
   padding: 2px 8px;
   border: 1px solid var(--border-color);
@@ -800,13 +867,11 @@ body.dd-resizing .dd-files-resizer { background: linear-gradient(90deg, transpar
   color: var(--accent-cyan);
   font-family: var(--font-mono);
   font-size: 0.68rem;
+  white-space: nowrap;
   cursor: pointer;
 }
 .dd-viewer-btn:hover { border-color: var(--accent-cyan); background: var(--accent-cyan-dim); }
-.dd-viewer-back { font-size: 0.8rem; }
-.dd-viewer-stale { padding: 4px 10px; background: var(--accent-amber-dim); color: var(--accent-amber); font-size: 0.72rem; }
-.dd-viewer-stale[hidden] { display: none; }
-.dd-viewer-content { display: flex; flex-direction: column; flex: 1; min-height: 0; overflow: hidden; }
+.dd-viewer-content { display: flex; flex: 1; min-height: 0; flex-direction: column; overflow: hidden; }
 .dd-viewer-content > .cm-editor { flex: 1; min-height: 0; }
 .dd-viewer-content .cm-editor.cm-focused { outline: none; }
 .dd-viewer-content.is-image {
@@ -820,13 +885,42 @@ body.dd-resizing .dd-files-resizer { background: linear-gradient(90deg, transpar
 .dd-md-frame { flex: 1; width: 100%; border: 0; background: var(--bg-terminal); }
 .dd-plain { flex: 1; margin: 0; padding: 10px; overflow: auto; font-size: 0.76rem; white-space: pre; }
 
+/* empty editor */
+.dd-editor-empty { display: flex; flex: 1; flex-direction: column; gap: 14px; padding: 28px 24px; overflow: auto; }
+.dd-editor-empty[hidden] { display: none; }
+.dd-empty-title { color: var(--text-secondary); font-size: 0.82rem; }
+.dd-empty-actions { display: flex; flex-wrap: wrap; gap: 6px; }
+.dd-empty-actions .dd-viewer-btn { padding: 5px 12px; font-size: 0.72rem; }
+.dd-empty-label { margin-bottom: 4px; color: var(--accent-orange); font-size: 0.62rem; letter-spacing: 0.12em; text-transform: uppercase; }
+.dd-recent { display: flex; align-items: center; gap: 6px; width: 100%; padding: 3px 4px; border: 0; background: none; color: var(--accent-cyan); font-family: var(--font-mono); font-size: 0.76rem; text-align: left; cursor: pointer; }
+.dd-recent:hover { background: var(--accent-cyan-dim); }
+.dd-recent-name { white-space: nowrap; }
+.dd-empty-hint { color: var(--text-muted); font-size: 0.68rem; }
+
+/* Narrow pane (phone, or dragged small): the explorer slides over the editor
+   and closes when a file opens; ☰ brings it back. */
+.dd-files.is-narrow .dd-explorer {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: 0;
+  z-index: 4;
+  width: min(300px, 85%);
+  box-shadow: 8px 0 24px rgba(0, 0, 0, 0.45);
+}
+html[data-theme="light"] .dd-files.is-narrow .dd-explorer { box-shadow: 8px 0 24px rgba(50, 80, 130, 0.25); }
+
 @media (max-width: 1099px) {
-  /* Narrow: the pane covers the terminal instead of squeezing it (squeezing
-     would resize tmux for every client). */
+  /* Narrow window: the pane covers the terminal instead of squeezing it
+     (squeezing would resize tmux for every client). */
   .dd-files { position: absolute; inset: var(--dd-gap); z-index: 18; flex: none; }
   .dd-files-resizer { display: none; }
 }
 @media (max-width: 719px) {
   .dd-cmd-btn { display: none; }
-  .dd-file-age { display: none; }
+  .dd-tab { max-width: 160px; }
+}
+@media (pointer: coarse) {
+  .dd-tree-row { min-height: 34px; }
+  .dd-tab-close { opacity: 0.7; width: 28px; height: 28px; }
 }