feat: add Files tab for browsing workspace files (#154)

* feat: add Files tab for browsing workspace files

- Add Files tab between Diff and Terminal in header
- Implement hierarchical file tree with expand/collapse
- Add fuzzy search with debouncing and relevance ranking
- Support gitignore filtering via `git check-ignore` (web + desktop)
- Add syntax highlighting for 150+ file types
- Add image preview (SVG, PNG, JPG, etc.)
- Add line numbers, wrap toggle, and copy button
- Desktop: split-pane layout matching DiffView
- Mobile: drill-in navigation with full-width sidebar
- Update help dialog with Cmd+3 shortcut
- Increase header breakpoint to 940px for new tab

* feat: enhance context and session stores to track agent/model/variant choices for historical sessions

* feat: implement line selection and commenting functionality in FilesView

packages/ui/src/index.css

@@ -920,7 +920,7 @@ html:not(.dark) .chat-scroll {
   display: inline;
 }
 
-@media (max-width: 820px) {
+@media (max-width: 940px) {
   :root:not(.mobile-pointer):not(.vscode-runtime) .header-tab-label {
     display: none;
   }