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/src/ui.js

@@ -47,7 +47,7 @@ toast.info = (message, duration) => toast(message, 'info', duration);
 // ---------------------------------------------------------------- dialogs
 
 let openDialogs = 0;
-export const dialogOpen = () => openDialogs > 0 || document.getElementById('lt-cmd-overlay')?.classList.contains('is-open');
+export const dialogOpen = () => openDialogs > 0 || !!document.querySelector('.lt-cmd-overlay.is-open');
 
 // A .lt-modal dialog. `body` is a node; `actions` are { label, value, primary,
 // danger }. Resolves with the chosen action's value (or `submit()`'s value when
@@ -153,22 +153,26 @@ function pushRecent(id) {
 }
 
 // Commands: { id, label, group, icon?, hint?, kbd?, keywords?, run }. `getCommands`
-// is called on every open, so the list reflects the current sessions and state.
+// (may be async) is called on every open, so the list reflects the current state.
+// Options: id (DOM prefix), placeholder, search(commands, query) -> ranked list
+// (replaces the default word filter, recents and grouping; used by Go to file).
 export class CommandPalette {
-  constructor(getCommands) {
+  constructor(getCommands, options = {}) {
+    this.options = { id: 'lt-cmd', placeholder: 'Type a command or a session name…', ...options };
+    const id = this.options.id;
     this.getCommands = getCommands;
     this.commands = [];
     this.visible = [];
     this.selected = 0;
     this.opener = null;
     this.input = el('input', {
-      class: 'lt-cmd-input', type: 'text', placeholder: 'Type a command or a session name…',
-      autocomplete: 'off', spellcheck: 'false', 'aria-label': 'Command', role: 'combobox',
-      'aria-controls': 'dd-cmd-results', 'aria-expanded': 'true',
+      class: 'lt-cmd-input', type: 'text', placeholder: this.options.placeholder,
+      autocomplete: 'off', spellcheck: 'false', 'aria-label': this.options.placeholder, role: 'combobox',
+      'aria-controls': `${id}-results`, 'aria-expanded': 'true',
     });
-    this.results = el('div', { class: 'lt-cmd-results', id: 'dd-cmd-results', role: 'listbox' });
-    this.overlay = el('div', { class: 'lt-cmd-overlay', id: 'lt-cmd-overlay', role: 'dialog', 'aria-modal': 'true', 'aria-label': 'Command palette' },
-      el('div', { class: 'lt-cmd-palette', id: 'lt-cmd-palette' },
+    this.results = el('div', { class: 'lt-cmd-results', id: `${id}-results`, role: 'listbox' });
+    this.overlay = el('div', { class: 'lt-cmd-overlay', id: `${id}-overlay`, role: 'dialog', 'aria-modal': 'true', 'aria-label': this.options.placeholder },
+      el('div', { class: 'lt-cmd-palette', id: `${id}-palette` },
         el('div', { class: 'lt-cmd-input-wrap' }, el('span', { class: 'lt-cmd-prompt', 'aria-hidden': 'true' }, '>'), this.input),
         this.results,
         el('div', { class: 'lt-cmd-footer' },
@@ -187,13 +191,29 @@ export class CommandPalette {
     return this.overlay.classList.contains('is-open');
   }
 
-  open(query = '') {
+  async open(query = '') {
     this.opener = document.activeElement;
-    this.commands = this.getCommands();
     this.overlay.classList.add('is-open');
     this.input.value = query;
-    this.render();
     this.input.focus();
+    const pending = this.getCommands();
+    if (pending instanceof Promise) {
+      this.commands = [];
+      this.loading = true;
+      this.render();
+      try {
+        this.commands = await pending;
+      } catch (error) {
+        this.commands = [];
+        this.results.replaceChildren(el('div', { class: 'lt-cmd-empty' }, error.message));
+        return;
+      } finally {
+        this.loading = false;
+      }
+    } else {
+      this.commands = pending;
+    }
+    if (this.isOpen) this.render();
   }
 
   close({ restoreFocus = true } = {}) {
@@ -246,6 +266,15 @@ export class CommandPalette {
   }
 
   render() {
+    if (this.loading) {
+      this.visible = [];
+      this.results.replaceChildren(el('div', { class: 'lt-cmd-empty' }, 'Loading…'));
+      return;
+    }
+    if (this.options.search) {
+      this.renderList(this.options.search(this.commands, this.input.value.trim()));
+      return;
+    }
     const words = this.input.value.trim().toLowerCase().split(/\s+/).filter(Boolean);
     const matches = this.commands.filter((c) => {
       const text = `${c.label} ${c.group} ${c.keywords ?? ''} ${c.hint ?? ''}`.toLowerCase();
@@ -266,14 +295,29 @@ export class CommandPalette {
       this.results.replaceChildren(el('div', { class: 'lt-cmd-empty' }, `No command matches “${this.input.value.trim()}”`));
       return;
     }
+    this.renderGroups(groups);
+  }
+
+  // A ranked list without group headers (custom search).
+  renderList(list) {
+    this.visible = list;
+    this.selected = 0;
+    if (!list.length) {
+      this.results.replaceChildren(el('div', { class: 'lt-cmd-empty' }, this.input.value.trim() ? `Nothing matches “${this.input.value.trim()}”` : 'Nothing here'));
+      return;
+    }
+    this.renderGroups(new Map([['', list]]));
+  }
+
+  renderGroups(groups) {
     let index = 0;
     const nodes = [];
     for (const [group, commands] of groups) {
-      nodes.push(el('div', { class: 'lt-cmd-group-label', role: 'presentation' }, group));
+      if (group) nodes.push(el('div', { class: 'lt-cmd-group-label', role: 'presentation' }, group));
       for (const command of commands) {
         const i = index;
         nodes.push(el('div', {
-          class: 'lt-cmd-item', id: `dd-cmd-${i}`, role: 'option',
+          class: 'lt-cmd-item', id: `${this.options.id}-item-${i}`, role: 'option',
           onclick: () => this.run(command),
           onmousemove: () => {
             if (this.selected !== i) {