feat: Add tiptap editor for task descriptions in slide-over panel

- Add updateTask tRPC mutation (name, description fields)
- Replace static description with TiptapEditor in TaskSlideOver
- Auto-detect existing markdown descriptions and convert to tiptap JSON
- Debounced auto-save with flush on close/unmount
- Saving indicator in header

docs/server-api.md

@@ -73,6 +73,7 @@ Each procedure uses `require*Repository(ctx)` helpers that throw `TRPCError(INTE
 | listTasks | query | Child tasks of parent |
 | getTask | query | Single task |
 | updateTaskStatus | mutation | Change task status |
+| updateTask | mutation | Update task fields (name, description) |
 | createInitiativeTask | mutation | Create task on initiative |
 | createPhaseTask | mutation | Create task on phase |
 | listInitiativeTasks | query | All tasks for initiative |