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/engine.ts

@@ -133,6 +133,7 @@ export class DocSyncEngine {
   private lockedByOthers: LeaseInfo[] = [];
   private conflicts: Result[] = [];
   private taskLinks = new Map<string, string>();
+  private subtaskLinks = new Map<string, string>();
 
   private focused: string | null = null;
   private activity = new Map<string, number>(); // blockId -> last edit/focus time
@@ -234,7 +235,7 @@ export class DocSyncEngine {
       errorMessage: this.errorMessage,
       role: this.role,
       title: this.local.title,
-      blocks: toDocumentBlocks(this.local, this.docId, this.taskLinks),
+      blocks: toDocumentBlocks(this.local, this.docId, this.taskLinks, this.subtaskLinks),
       pendingCount: this.pending.length + (this.inflight?.length || 0),
       shared: this.shared,
       sessions: this.sessions,
@@ -252,8 +253,9 @@ export class DocSyncEngine {
   }
 
   /** Task IDs of blocks linked to tasks (kept outside the synced tree). */
-  setTaskLinks(links: Map<string, string>): void {
+  setTaskLinks(links: Map<string, string>, subtaskLinks?: Map<string, string>): void {
     this.taskLinks = new Map(links);
+    if (subtaskLinks) this.subtaskLinks = new Map(subtaskLinks);
     this.emit();
   }
 
@@ -562,6 +564,9 @@ export class DocSyncEngine {
       this.role = res.role;
       this.shared = res.shared;
       this.sessions = res.sessions;
+      // The server reports the links after every sync, so badges follow subtasks it adopted or added
+      if (res.taskLinks) this.taskLinks = new Map(Object.entries(res.taskLinks));
+      if (res.subtaskLinks) this.subtaskLinks = new Map(Object.entries(res.subtaskLinks));
       this.presence = res.presence || [];
       this.lockedByOthers = res.leases?.others || [];