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/domain/models.go

@@ -242,6 +242,8 @@ type SubTask struct {
 	Title       string `json:"title"`
 	IsCompleted bool   `json:"isCompleted"`
 	CreatedAt   string `json:"createdAt,omitempty"`
+	// BlockID: the note checkbox this subtask is tied to, when its task is linked to a note
+	BlockID string `json:"blockId,omitempty"`
 }
 
 type Thought struct {
@@ -449,6 +451,7 @@ type DocumentBlock struct {
 	Position   float64 `json:"position"`
 	Collapsed  bool    `json:"collapsed,omitempty"`
 	TaskID     string  `json:"taskId,omitempty"`
+	SubtaskOf  string  `json:"subtaskOf,omitempty"` // checkbox tied to a subtask of this task
 }
 
 func CompileBlocksToMarkdown(blocks []DocumentBlock) string {