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/git.ts

@@ -10,6 +10,7 @@ import type {
   GitCommitComparison,
   GitRepositoryFile,
   GitSearchHit,
+  GitStash,
   GitStatus,
   LocalModelOption,
   PatchApplyAction,
@@ -47,6 +48,10 @@ export function listBranches(path: string): Promise<GitBranch[]> {
   return invoke<GitBranch[]>("list_branches", { path });
 }
 
+export function listStashes(path: string): Promise<GitStash[]> {
+  return invoke<GitStash[]>("list_stashes", { path });
+}
+
 export function checkoutBranch(path: string, branch: string): Promise<GitStatus> {
   return invoke<GitStatus>("checkout_branch", { path, branch });
 }
@@ -104,6 +109,30 @@ export function commit(path: string, message: string): Promise<GitStatus> {
   return invoke<GitStatus>("commit", { path, message });
 }
 
+export function stashPush(
+  path: string,
+  message?: string,
+  includeUntracked = true,
+): Promise<GitStatus> {
+  return invoke<GitStatus>("stash_push", {
+    path,
+    message: message?.trim() ? message.trim() : null,
+    includeUntracked,
+  });
+}
+
+export function stashApply(path: string, selector: string): Promise<GitStatus> {
+  return invoke<GitStatus>("stash_apply", { path, selector });
+}
+
+export function stashPop(path: string, selector: string): Promise<GitStatus> {
+  return invoke<GitStatus>("stash_pop", { path, selector });
+}
+
+export function stashDrop(path: string, selector: string): Promise<GitStatus> {
+  return invoke<GitStatus>("stash_drop", { path, selector });
+}
+
 export function commitAiStatus(): Promise<CommitAiStatus> {
   return invoke<CommitAiStatus>("commit_ai_status");
 }