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/stores/useUIStore.ts

@@ -4,7 +4,7 @@ import type { SidebarSection } from '@/constants/sidebar';
 import { getSafeStorage } from './utils/safeStorage';
 import { SEMANTIC_TYPOGRAPHY, getTypographyVariable, type SemanticTypographyKey } from '@/lib/typography';
 
-export type MainTab = 'chat' | 'git' | 'diff' | 'terminal';
+export type MainTab = 'chat' | 'git' | 'diff' | 'terminal' | 'files';
 export type EventStreamStatus =
   | 'idle'
   | 'connecting'