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/OutlinerBlock.tsx

@@ -1488,6 +1488,24 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
             </button>
           </div>
         )}
+        {/* Checkbox tied to a subtask of a linked task: tick and text follow the subtask */}
+        {block.type === 'todo' && !block.taskId && block.subtaskOf && (
+          <div className="flex items-center gap-1 mt-0.5 select-none">
+            <button
+              type="button"
+              onMouseDown={(e) => e.stopPropagation()}
+              onClick={(e) => {
+                e.stopPropagation();
+                onOpenTask?.(block.subtaskOf as string);
+              }}
+              className="badge-neutral text-[9px] inline-flex items-center gap-0.5 px-1.5 py-0.5 rounded font-medium shrink-0 cursor-pointer hover:opacity-80"
+              title="Unteraufgabe einer verknüpften Aufgabe – Aufgabe öffnen"
+            >
+              <CheckSquare className="w-2.5 h-2.5" />
+              <span>Unteraufgabe</span>
+            </button>
+          </div>
+        )}
 
         {/* Conflict copy: text that lost against someone else's change */}
         {block.conflictOf && (
@@ -1619,7 +1637,9 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
                 className="absolute right-0 top-full mt-1 bg-panel border border-border shadow-lg rounded-card p-1 z-30 w-52 text-xs"
                 onClick={(e) => e.stopPropagation()}
               >
-                {!block.taskId ? (
+                {block.subtaskOf && !block.taskId ? (
+                  <p className="px-2 py-1.5 text-faint">Unteraufgabe einer verknüpften Aufgabe</p>
+                ) : !block.taskId ? (
                   <button
                     type="button"
                     onClick={async () => {