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/capabilities/default.json

@@ -17,9 +17,12 @@
     "dialog:default",
     "shell:default",
     "fs:default",
+    "fs:read-all",
+    "fs:write-all",
     "core:window:allow-set-size",
     "core:window:allow-set-position",
     "core:window:allow-outer-size",
-    "core:window:allow-outer-position"
+    "core:window:allow-outer-position",
+    "notification:default"
   ]
 }