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/components/layout/MainLayout.tsx

@@ -16,7 +16,7 @@ import { useDeviceInfo } from '@/lib/device';
 import { useEdgeSwipe } from '@/hooks/useEdgeSwipe';
 import { cn } from '@/lib/utils';
 
-import { ChatView, GitView, DiffView, TerminalView, SettingsView } from '@/components/views';
+import { ChatView, GitView, DiffView, TerminalView, FilesView, SettingsView } from '@/components/views';
 
 export const MainLayout: React.FC = () => {
     const {
@@ -306,6 +306,8 @@ export const MainLayout: React.FC = () => {
                 return <DiffView />;
             case 'terminal':
                 return <TerminalView />;
+            case 'files':
+                return <FilesView />;
             default:
                 return null;
         }