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/features/workspace/DocumentEditor.tsx

@@ -128,6 +128,7 @@ function sameBlocks(a: DocumentBlock[], b: DocumentBlock[]): boolean {
       !!x.collapsed !== !!y.collapsed ||
       (x.lang || '') !== (y.lang || '') ||
       (x.taskId || '') !== (y.taskId || '') ||
+      (x.subtaskOf || '') !== (y.subtaskOf || '') ||
       (x.conflictOf || '') !== (y.conflictOf || '')
     ) {
       return false;
@@ -360,10 +361,12 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
           setBlocks(normalized);
         }
         const links = new Map<string, string>();
+        const subtaskLinks = new Map<string, string>();
         for (const b of fetched.blocks || []) {
           if (b.taskId) links.set(b.id, b.taskId);
+          if (b.subtaskOf) subtaskLinks.set(b.id, b.subtaskOf);
         }
-        engineRef.current?.setTaskLinks(links);
+        engineRef.current?.setTaskLinks(links, subtaskLinks);
         setLoading(false);
         onDocMetadataChange(fetched);