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/Cargo.toml

@@ -23,6 +23,7 @@ tauri-plugin-opener = "2"
 tauri-plugin-fs = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-shell = "2"
+tauri-plugin-notification = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"