Error messages not showing in toolbar (vibe-kanban) (#274)

* ## Summary

I've successfully fixed the error message display issue. Here's what I found and changed:

**The Problem:**
- The backend was correctly returning detailed error messages in the API response format: `{"success": false, "data": null, "message": "Git service error: Merge conflicts: Rebase failed due to conflicts. Please resolve conflicts manually."}`
- The frontend's API layer was correctly parsing these errors and throwing `ApiError` objects with the proper message
- However, the rebase handlers in [`CurrentAttempt.tsx`](file:///private/var/folders/kr/jdxkcn7129j376nrg0stj9zm0000gn/T/vibe-kanban/vk-30d8-error-mess/frontend/src/components/tasks/Toolbar/CurrentAttempt.tsx#L349) were catching these errors and replacing them with generic "Failed to rebase branch" messages

**The Solution:**
- Updated both rebase error handlers to preserve the actual error message: `setError(err instanceof Error ? err.message : 'Failed to rebase branch')`
- This ensures that detailed error messages like "Git service error: Merge conflicts: Rebase failed due to conflicts. Please resolve conflicts manually." will now be displayed to users instead of the generic message

**Result:**
- Users will now see helpful, specific error messages when rebase operations fail
- The response structure is consistent (`success: false` with detailed `message`)
- Both frontend and backend build successfully

The fix is minimal and focused - it preserves the existing API structure while ensuring error messages flow through to the UI properly.

* fmt

* clippy

frontend/src/components/tasks/Toolbar/CurrentAttempt.tsx

@@ -346,7 +346,7 @@ function CurrentAttempt({
       // Refresh branch status after rebase
       fetchBranchStatus();
     } catch (err) {
-      setError('Failed to rebase branch');
+      setError(err instanceof Error ? err.message : 'Failed to rebase branch');
     } finally {
       setRebasing(false);
     }
@@ -367,7 +367,7 @@ function CurrentAttempt({
       fetchBranchStatus();
       setShowRebaseDialog(false);
     } catch (err) {
-      setError('Failed to rebase branch');
+      setError(err instanceof Error ? err.message : 'Failed to rebase branch');
     } finally {
       setRebasing(false);
     }