feat(tasks): linked tasks sync title and subtasks with their note in both directions

Subtasks of a task linked to a note checkbox are tied 1:1 to checkboxes of that note
(subtasks[].blockId), wherever they are moved. ReconcileDocTaskLinks now carries tick, title and
subtasks from the note to the task and adopts new checkboxes directly under the task;
SyncTaskToDoc carries tick, title and subtasks to the note and adds checkboxes for new subtasks.
Deleting is not mirrored. Deepening a task ties its subtasks right away; saves without links keep
the server's links. The sync response reports taskLinks/subtaskLinks; the editor shows an
"Unteraufgabe" badge. v0.36.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/docSync/transport.ts

@@ -51,6 +51,9 @@ export interface SyncResponse {
   sessions: number;
   presence: Presence[];
   leases: { granted: string[]; denied: string[]; others: LeaseInfo[] };
+  // Checkboxes linked to a task / to a subtask (block → task), for the badges
+  taskLinks?: Record<string, string>;
+  subtaskLinks?: Record<string, string>;
 }
 
 /** status 0 = network error or timeout; otherwise the HTTP status. */