feat(git stash): add stash listing and push/apply/pop/drop UI

This change introduces Git stash support end-to-end, including a new
backend command to list stashes and operations to push, apply, pop, and
drop them. The frontend now fetches stashes as part of the repository
bundle and provides a dedicated panel to manage shelved changes.

- Add GitStash model and Tauri commands for stash operations
- Implement StashPanel component and wire it into the app
- Adjust sidebar layout and add stash-specific styling

src/lib/types.ts

@@ -58,6 +58,15 @@ export interface GitBranch {
   remote: boolean;
 }
 
+export interface GitStash {
+  selector: string;
+  index: number;
+  hash: string;
+  branch: string | null;
+  message: string;
+  date: string;
+}
+
 export interface GitCommit {
   hash: string;
   short_hash: string;
@@ -85,6 +94,7 @@ export interface GitRepositoryFile {
 export interface RepositoryBundle {
   status: GitStatus;
   branches: GitBranch[];
+  stashes: GitStash[];
   commits: GitCommit[];
   files: GitRepositoryFile[];
 }