feat: v0.8.0 edit and save files with conflict detection

- devdesk attach --fs-write SESSION PATH BYTES BASESHA|new: streamed like
  uploads, temp file in the same folder, sha re-check (conflict reply instead
  of overwriting), mode/owner kept, atomic mv; new files with folders, never
  outside the session folder or through a symlink out of it.
- Editor: editable CodeMirror for UTF-8 text, dirty tracking, Ctrl+S,
  Save/Revert, conflict dialog with a disk-vs-yours diff, changed-on-disk
  banner, drafts in localStorage (flushed on pagehide), close/leave guards,
  New file.
- Upload streaming factored into streamIn(), shared by uploads and saves.

Closes #20

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

web/styles.css

@@ -924,3 +924,26 @@ html[data-theme="light"] .dd-files.is-narrow .dd-explorer { box-shadow: 8px 0 24
   .dd-tree-row { min-height: 34px; }
   .dd-tab-close { opacity: 0.7; width: 28px; height: 28px; }
 }
+
+/* ---- editing (devdesk#20) ------------------------------------------------------------- */
+.dd-tab-view:not(.is-dirty) .dd-dirty-only { display: none; }
+.dd-unsaved { color: var(--accent-amber); }
+.dd-tab.is-dirty .dd-tab-close { opacity: 1; color: var(--accent-amber); }
+.dd-tab.is-dirty .dd-tab-close:hover { color: var(--accent-red); }
+.dd-viewer-btn.is-primary { border-color: var(--accent-orange); color: var(--accent-orange); }
+.dd-viewer-btn.is-primary:hover { background: var(--accent-orange-dim); }
+.dd-viewer-banner {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 6px 10px;
+  padding: 5px 10px;
+  border-bottom: 1px solid var(--accent-amber-dim);
+  background: var(--accent-amber-dim);
+  color: var(--accent-amber);
+  font-size: 0.72rem;
+}
+.dd-dialog.is-wide .lt-modal { width: min(1100px, 94vw); max-height: 88vh; }
+.dd-dialog.is-wide .lt-modal-body { padding: 0; }
+.dd-compare { display: flex; height: min(64vh, 640px); }
+.dd-compare > .cm-editor { flex: 1; min-width: 0; }