feat: Phase 3 - filter bar, keyboard navigation, notifications, comments

- FilterBar component with text search, label chips, due date and priority dropdowns
- "/" keyboard shortcut and toolbar button to toggle filter bar
- Keyboard card navigation with J/K/H/L keys, Enter to open, Escape to clear
- Focus ring on keyboard-selected cards with auto-scroll
- Desktop notifications for due/overdue cards via tauri-plugin-notification
- CommentsSection component with add/delete and relative timestamps
- Filtered card count display in column headers

src-tauri/src/lib.rs

@@ -17,6 +17,7 @@ pub fn run() {
         .plugin(tauri_plugin_fs::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_shell::init())
+        .plugin(tauri_plugin_notification::init())
         .setup(|app| {
             // Get portable data directory next to the exe
             let exe_path =