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/storage/documents.go

@@ -12,9 +12,10 @@ import (
 func (s *Store) loadDocBlocks(docID string, fallbackJSON string) []domain.DocumentBlock {
 	if d, _, err := loadCollabDoc(s.db, docID); err == nil && len(d.Blocks) > 0 {
 		blocks := flatDocumentBlocks(docID, d)
-		if links := s.docTaskLinks(docID); len(links) > 0 {
+		if links, subLinks := s.DocTaskLinks(docID); len(links) > 0 {
 			for i := range blocks {
 				blocks[i].TaskID = links[blocks[i].ID]
+				blocks[i].SubtaskOf = subLinks[blocks[i].ID]
 			}
 		}
 		return blocks