feat(notes): browser sync engine with offline queue; editor saves through the operation log

Phase 2 of the collaboration plan:
- frontend/src/lib/docSync: TypeScript port of the block tree, operations,
  effects and order keys (kept identical to Go via shared test vectors),
  diff from the editor's flat list to operations, IndexedDB persistence,
  HTTP transport and DocSyncEngine (one request in flight, idempotent
  resends, backoff, 3 s/30 s polling, keepalive flush and leave on exit,
  block leases for focus, 20 s grace and new blocks).
- Text changes carry the version the user saw when typing (first change
  wins when coalescing; a stale focused field keeps its old base), so a
  concurrent edit becomes a conflict copy instead of a silent overwrite.
- DocumentEditor uses the engine instead of the 800 ms whole-document PUT;
  editors (not only owners) can edit, remote changes appear live.
- Server: task IDs of linked blocks are delivered again, linked tasks follow
  ticked/deleted blocks after sync, note tags are derived, title changes
  notify other users; block IDs already used in another note are renamed
  (starter content with fixed IDs was empty for every further user).
- Fix: unlinking tasks from blocks wrote to non-existent columns and never
  worked (error was swallowed).
- Tests: Go/TS order key vectors, diff round trips, engine races (lost
  responses, offline + reload, edits arriving between typing and sending,
  stale focused field, duplicate first block), plus a Playwright test with
  two browsers against the real image (e2e/run.sh).

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

internal/storage/docsync.go

@@ -355,9 +355,21 @@ func persistDocState(tx *sql.Tx, docID string, before, after *collab.Doc, head i
 		}
 	}
 	blocksJSON, _ := json.Marshal(flat)
+	tagSet := map[string]bool{}
+	for _, b := range flat {
+		for _, tag := range domain.ExtractTags(b.Content) {
+			tagSet[tag] = true
+		}
+	}
+	tags := make([]string, 0, len(tagSet))
+	for tag := range tagSet {
+		tags = append(tags, tag)
+	}
+	sort.Strings(tags)
+	tagsJSON, _ := json.Marshal(tags)
 	if _, err := tx.Exec(
-		`UPDATE documents SET title = ?, title_version = ?, head_seq = ?, markdown_cache = ?, content = ?, blocks_json = ?, updated_at = ? WHERE id = ?`,
-		after.Title, after.TitleVersion, head, domain.CompileBlocksToMarkdown(flat), plain.String(), string(blocksJSON), now, docID,
+		`UPDATE documents SET title = ?, title_version = ?, head_seq = ?, markdown_cache = ?, content = ?, blocks_json = ?, tags = ?, updated_at = ? WHERE id = ?`,
+		after.Title, after.TitleVersion, head, domain.CompileBlocksToMarkdown(flat), plain.String(), string(blocksJSON), string(tagsJSON), now, docID,
 	); err != nil {
 		return err
 	}
@@ -377,6 +389,71 @@ func persistDocState(tx *sql.Tx, docID string, before, after *collab.Doc, head i
 	return nil
 }
 
+// docTaskLinks maps block IDs of a note to the IDs of tasks created from them ("Erweitert").
+func (s *Store) docTaskLinks(docID string) map[string]string {
+	links := map[string]string{}
+	rows, err := s.db.Query(
+		`SELECT id, converted_ref_id FROM thoughts WHERE converted_type = 'document_block' AND converted_ref_id LIKE ?`,
+		docID+":%")
+	if err != nil {
+		return links
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var taskID, ref string
+		if rows.Scan(&taskID, &ref) == nil {
+			if _, blockID, ok := strings.Cut(ref, ":"); ok {
+				links[blockID] = taskID
+			}
+		}
+	}
+	return links
+}
+
+// ReconcileDocTaskLinks brings tasks linked to blocks in line with the note: a ticked block completes its
+// task, a deleted block unlinks it (the task itself stays). Returns the IDs of changed tasks.
+func (s *Store) ReconcileDocTaskLinks(docID string) []string {
+	links := s.docTaskLinks(docID)
+	if len(links) == 0 {
+		return nil
+	}
+	d, _, err := loadCollabDoc(s.db, docID)
+	if err != nil {
+		return nil
+	}
+	var changed []string
+	for blockID, taskID := range links {
+		b, ok := d.Blocks[blockID]
+		if !ok || b.Deleted {
+			if _, err := s.db.Exec(`UPDATE thoughts SET converted_type = '', converted_ref_id = '' WHERE id = ?`, taskID); err == nil {
+				changed = append(changed, taskID)
+			}
+			continue
+		}
+		kanban := "todo"
+		if b.Props.Checked {
+			kanban = "done"
+		}
+		res, err := s.db.Exec(
+			`UPDATE thoughts SET is_completed = ?, kanban_status = ? WHERE id = ? AND is_completed != ?`,
+			boolInt(b.Props.Checked), kanban, taskID, boolInt(b.Props.Checked))
+		if err == nil {
+			if n, _ := res.RowsAffected(); n > 0 {
+				changed = append(changed, taskID)
+			}
+		}
+	}
+	sort.Strings(changed)
+	return changed
+}
+
+// GetThoughtByIDUnchecked loads a task without access check, for server-internal notifications.
+func (s *Store) GetThoughtByIDUnchecked(thoughtID string) (*domain.Thought, error) {
+	return scanThought(s.db.QueryRow(
+		`SELECT id, user_id, content, tags, priority, source, status, is_completed, converted_type, converted_ref_id, due_date, task_list_id, kanban_status, notes, alarm_sent, created_at, triaged_at, subtasks, sort_order
+		 FROM thoughts WHERE id = ?`, thoughtID))
+}
+
 // GetDocOpsSince returns up to limit log entries after seq, plus the current head.
 func (s *Store) GetDocOpsSince(docID string, since int64, limit int) ([]DocLogEntry, int64, error) {
 	var head int64
@@ -495,12 +572,36 @@ const legacyClientID = "legacy"
 // replaceDocBlocksInDB stores a complete flat block list by diffing it against the current tree and
 // applying the difference as regular operations, so it shows up in the log like any other change.
 func (s *Store) replaceDocBlocksInDB(docID, userID string, blocks []domain.DocumentBlock, _ string) {
+	blocks = s.renameForeignBlockIDs(docID, blocks)
 	a := collab.Actor{UserID: userID, ClientID: legacyClientID}
 	if _, _, err := s.applyDocChange(docID, a, nil, func(d *collab.Doc) []collab.Op { return legacyOps(d, blocks) }); err != nil {
 		slog.Error("Speichern der Notiz fehlgeschlagen", "doc_id", docID, "error", err)
 	}
 }
 
+// renameForeignBlockIDs gives new IDs to blocks whose ID already belongs to another note (e.g. fixed IDs of
+// starter content or copied blocks). Block IDs are global, so keeping them would make the insert fail.
+func (s *Store) renameForeignBlockIDs(docID string, blocks []domain.DocumentBlock) []domain.DocumentBlock {
+	out := blocks
+	copied := false
+	for i, b := range blocks {
+		id := strings.TrimSpace(b.ID)
+		if id == "" {
+			continue
+		}
+		var owner string
+		if s.db.QueryRow(`SELECT document_id FROM blocks WHERE id = ?`, id).Scan(&owner) != nil || owner == docID {
+			continue
+		}
+		if !copied {
+			out = append([]domain.DocumentBlock(nil), blocks...)
+			copied = true
+		}
+		out[i].ID = domain.NewID("blk")
+	}
+	return out
+}
+
 // applyLegacyTitle records a title change from the whole-document API in the log.
 func (s *Store) applyLegacyTitle(docID, userID, title string) {
 	a := collab.Actor{UserID: userID, ClientID: legacyClientID}