feat(FN-803): add ahead/behind commit lists to Git Manager Remotes tab

- Add backend API routes for remote commit lists (local-ahead, remote-ahead, all commits by branch)
- Surface ahead/behind commit counts and expandable commit lists in Remotes tab UI
- Add expand/collapse rows showing commit hash, message, author, and relative date
- Style commit list rows with alternating backgrounds and monospace hashes
- Add comprehensive tests for API routes and GitManagerModal component
- Update README documentation for Git Manager Remotes tab

README.md

@@ -379,6 +379,7 @@ Built-in Git repository visualization and management:
 - Manage branches
 - See worktree/task associations
 - Perform fetch/pull/push operations
+- View pending-push commits and inspect recent commit history per remote
 
 ### Activity Log