feat: add task comments feature with CRUD operations

- Introduced a new `task_comments` table in the database schema.
- Implemented task comments API endpoints for listing, creating, and deleting comments.
- Enhanced the task detail dialog to display comments and allow users to add new comments.
- Updated task row component to handle click events for viewing task details.
- Added a theme provider to manage light/dark mode across the application.
- Refactored Milkdown editor to use Crepe for improved markdown editing experience.
- Updated global styles to accommodate new editor and theme changes.
- Enhanced task filtering and sorting functionality in the tasks page.

prd.json

@@ -333,8 +333,8 @@
         "Typecheck passes"
       ],
       "priority": 18,
-      "passes": false,
-      "notes": ""
+      "passes": true,
+      "notes": "Provider-agnostic architecture: AIProvider interface + registry in src/main/ai/provider.ts allows swapping AI backends (Copilot, OpenAI, Anthropic, etc.) without changing token storage or UI. Token keyed by provider name in OS keychain via keytar."
     },
     {
       "id": "US-019",