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/Header.tsx

@@ -5,7 +5,7 @@ import {
   TooltipTrigger,
 } from '@/components/ui/tooltip';
 
-import { RiChat4Line, RiCodeLine, RiCommandLine, RiGitBranchLine, RiLayoutLeftLine, RiPlayListAddLine, RiQuestionLine, RiSettings3Line, RiTerminalBoxLine, type RemixiconComponentType } from '@remixicon/react';
+import { RiChat4Line, RiCodeLine, RiCommandLine, RiFolder6Line, RiGitBranchLine, RiLayoutLeftLine, RiPlayListAddLine, RiQuestionLine, RiSettings3Line, RiTerminalBoxLine, type RemixiconComponentType } from '@remixicon/react';
 import { useUIStore, type MainTab } from '@/stores/useUIStore';
 import { useUpdateStore } from '@/stores/useUpdateStore';
 import { useConfigStore } from '@/stores/useConfigStore';
@@ -266,6 +266,7 @@ export const Header: React.FC = () => {
       icon: RiCodeLine,
       badge: !isMobile && diffFileCount > 0 ? diffFileCount : undefined,
     },
+    { id: 'files', label: 'Files', icon: RiFolder6Line },
     { id: 'terminal', label: 'Terminal', icon: RiTerminalBoxLine },
     {
       id: 'git',