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>

internal/api/docsync.go

@@ -39,6 +39,9 @@ type syncResponse struct {
 	Sessions int                   `json:"sessions"`
 	Presence []collab.Presence     `json:"presence"`
 	Leases   syncLeases            `json:"leases"`
+	// Checkboxes linked to a task or to a subtask (block → task), for the editor's badges
+	TaskLinks    map[string]string `json:"taskLinks"`
+	SubtaskLinks map[string]string `json:"subtaskLinks"`
 }
 
 // docAccess resolves the caller's role; it writes 404 and returns "" without access.
@@ -171,6 +174,7 @@ func (s *Server) handleDocSync(w http.ResponseWriter, r *http.Request) {
 	resp.HeadSeq = head
 	resp.HasMore = len(entries) == syncMaxLog && entries[len(entries)-1].Seq < head
 	resp.Shared = s.store.IsDocShared(docID)
+	resp.TaskLinks, resp.SubtaskLinks = s.store.DocTaskLinks(docID)
 	resp.Sessions, resp.Presence = s.leases.Sessions(docID)
 	resp.Leases.Others = s.leases.Others(docID, req.ClientID)