fix(db): report write errors, multi-step changes all or nothing (FLAW-14)

Store.execInTx runs related statements in one transaction and returns
the first error. Used for deleting and moving folders (with the trash),
shared folders, folder shares and mounts, deleting task lists and
accounts, pinning, and the password reset (new password and the end of
all other links together). Writes whose success the API reports now
pass their error to the client (logout, new password, setup, sharing
mode, new group, restore, unlinking a note block), follow-up tasks and
alarm markers log failures. Remaining ignored writes are listed as
intentional in flaws.md.

Found on the way: DeleteTaskList unlinked all tasks before checking
ownership, so a stranger with the list ID could empty it; viewers could
unlink tasks from note blocks. Both fixed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/thoughts.go

@@ -485,7 +485,9 @@ func (s *Store) UpdateThoughtDetails(userID, thoughtID string, content *string,
 	}
 	// A new due date or a reopened task gets its alarm again
 	if thought.DueDate != oldDue || (wasCompleted && !thought.IsCompleted) {
-		_, _ = s.db.Exec(`UPDATE thoughts SET alarm_sent = 0, alarm_failures = 0 WHERE id = ?`, thoughtID)
+		if _, err := s.db.Exec(`UPDATE thoughts SET alarm_sent = 0, alarm_failures = 0 WHERE id = ?`, thoughtID); err != nil {
+			return nil, err
+		}
 		thought.AlarmSent = false
 	}
 	if taskListID != nil {
@@ -538,8 +540,10 @@ func (s *Store) CreateTaskFromDocBlock(userID, docID, blockID, content string, c
 		doneInt = 1
 		t.KanbanStatus = "done"
 	}
-	_, _ = s.db.Exec(`UPDATE thoughts SET converted_type = 'document_block', converted_ref_id = ?, is_completed = ?, kanban_status = ?, status = 'kept' WHERE id = ?`,
-		t.ConvertedRefID, doneInt, t.KanbanStatus, t.ID)
+	if _, err := s.db.Exec(`UPDATE thoughts SET converted_type = 'document_block', converted_ref_id = ?, is_completed = ?, kanban_status = ?, status = 'kept' WHERE id = ?`,
+		t.ConvertedRefID, doneInt, t.KanbanStatus, t.ID); err != nil {
+		return nil, err
+	}
 
 	s.SetBlockChecked(userID, docID, blockID, checked)
 	return t, nil
@@ -561,11 +565,16 @@ func (s *Store) SyncDocBlockAndTask(userID, taskID string, isCompleted bool) str
 
 // UnlinkTaskFromDocBlock removes the link between a task and its note block; both stay. The block's
 // task ID is derived from this link (docTaskLinks), so nothing in the note itself needs to change.
-func (s *Store) UnlinkTaskFromDocBlock(userID, taskID string) {
-	if t, err := s.GetThoughtByID(userID, taskID); err != nil || t == nil {
-		return
+func (s *Store) UnlinkTaskFromDocBlock(userID, taskID string) error {
+	t, err := s.GetThoughtByID(userID, taskID)
+	if err != nil || t == nil {
+		return err
 	}
-	_, _ = s.db.Exec(`UPDATE thoughts SET converted_type = '', converted_ref_id = '' WHERE id = ?`, taskID)
+	if err := s.ensureTaskListWritable(userID, t.TaskListID); err != nil {
+		return err
+	}
+	_, err = s.db.Exec(`UPDATE thoughts SET converted_type = '', converted_ref_id = '' WHERE id = ?`, taskID)
+	return err
 }
 
 func (s *Store) SyncTaskFromBlockCheck(userID, taskID string, checked bool) {