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/lib/api/types.ts

@@ -339,11 +339,16 @@ export interface CommandExecResult {
   error?: string;
 }
 
+export interface ListDirectoryOptions {
+  respectGitignore?: boolean;
+}
+
 export interface FilesAPI {
-  listDirectory(path: string): Promise<DirectoryListResult>;
+  listDirectory(path: string, options?: ListDirectoryOptions): Promise<DirectoryListResult>;
   search(payload: FileSearchQuery): Promise<FileSearchResult[]>;
   createDirectory(path: string): Promise<{ success: boolean; path: string }>;
   readFile?(path: string): Promise<{ content: string; path: string }>;
+  readFileBinary?(path: string): Promise<{ dataUrl: string; path: string }>;
   writeFile?(path: string, content: string): Promise<{ success: boolean; path: string }>;
   execCommands?(commands: string[], cwd: string): Promise<{ success: boolean; results: CommandExecResult[] }>;
 }