feat: v0.7.0 files pane: browse, preview, diff, Claude's edits

- devdesk attach --fs-list/--fs-read/--git-status/--git-show/--asset
  (libexec/devdesk-files.sh), confined to the session folder.
- The state hook records files Claude edits (Edit/Write/MultiEdit/
  NotebookEdit) for the Changes tab's "C" badge; rename/end carry it.
- Files pane: folders, CodeMirror read-only view, sanitized markdown preview
  in a sandboxed iframe with inlined relative images, images, Changes tab with
  line counts and diff against HEAD, "→ Claude" @path, resizable split.
- The editor bundle ships as /usr/share/devdesk/assets/editor.js, is fetched
  over ttyd on first use, verified against the page's SHA-256 and cached in
  IndexedDB.
- Fix: the state hook's field split collapsed empty fields.

Closes #19

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

web/index.template.html

@@ -63,11 +63,13 @@
         </div>
         <button type="button" class="dd-menu-item" id="theme-menu" role="menuitem">Light mode</button>
         <button type="button" class="dd-menu-item" id="notify-toggle" role="menuitem">Desktop notifications</button>
+        <button type="button" class="dd-menu-item" id="palette-menu" role="menuitem">Command palette</button>
         <button type="button" class="dd-menu-item" id="help-open" role="menuitem">Keyboard shortcuts</button>
         <button type="button" class="dd-menu-item" id="fullscreen-toggle" role="menuitem" title="In Chromium, fullscreen also captures Ctrl+W/Ctrl+T for the terminal">Fullscreen</button>
         <div class="dd-menu-foot" id="version">devdesk</div>
       </div>
     </div>
+    <button type="button" class="lt-btn lt-btn-sm dd-files-toggle" id="files-toggle" aria-controls="files" aria-expanded="false" title="Files: browse, preview and diff this session's folder (Ctrl+Shift+E)"><span class="dd-panels-label">Files</span><span class="dd-panels-icon" aria-hidden="true">🗀</span></button>
     <button type="button" class="lt-btn lt-btn-sm dd-panels" id="sidebar-toggle" aria-controls="sidebar" aria-expanded="false"><span class="dd-panels-label">Panels</span><span class="dd-panels-icon" aria-hidden="true">▤</span></button>
   </div>
 </header>
@@ -88,6 +90,22 @@
     </div>
   </section>
 
+  <section class="lt-frame dd-files" id="files" aria-label="Files" hidden>
+    <div class="dd-files-resizer" id="files-resizer" role="separator" aria-orientation="vertical" aria-label="Resize the files pane" tabindex="0"></div>
+    <div class="dd-files-head">
+      <div class="dd-files-tabs" role="tablist">
+        <button type="button" role="tab" data-files-tab="files" aria-selected="true">Files</button>
+        <button type="button" role="tab" data-files-tab="changes" aria-selected="false">Changes <span class="dd-files-count"></span></button>
+      </div>
+      <span class="dd-files-actions">
+        <button type="button" class="dd-icon-btn dd-files-refresh" title="Refresh" aria-label="Refresh">↻</button>
+        <button type="button" class="dd-icon-btn dd-files-close" title="Close (Esc)" aria-label="Close files">✕</button>
+      </span>
+    </div>
+    <nav class="dd-files-crumbs" aria-label="Path"></nav>
+    <div class="dd-files-body" tabindex="-1"></div>
+  </section>
+
   <div class="dd-backdrop" id="backdrop" hidden></div>
   <aside id="sidebar" class="dd-sidebar" aria-label="Panels">
     <section class="lt-frame dd-widget" id="widget-sessions" data-state="live">