feat(notes): show block locks and presence, conflict copies with accept/discard, undo/redo

Phase 3 of the collaboration plan:
- Blocks held by someone else show a lock with the name and are read-only;
  Enter in such a block adds a new block below. Delete, merge, indent and
  drag are not started when a involved block (or one in its branch) is
  locked; a short notice explains why (fixes FLAW-25).
- Presence: the editor header lists who else has the note open.
- Conflict copies are highlighted ("Konflikt – Version von …") with
  Übernehmen (replaces the original or the title) and Verwerfen. Both check
  the locks of the original and of the copy first.
- Undo/redo of own changes (Ctrl/Cmd+Z, Ctrl+Shift+Z, Ctrl+Y, beforeinput
  history events): typing is grouped per block until a 1 s pause, up to 200
  steps per tab; a step is skipped with a notice if someone else changed or
  holds the block meanwhile. New clearConflict props patch on the server.
- New blocks get focus synchronously on mount (fixes FLAW-26).
- Fix: after reopening a note from local storage the engine polled every
  30 s instead of 3 s until the next change, because the delay was decided
  before the first response told it the note is shared.
- Tests: undo/conflict engine tests, lock/undo/conflict UI tests and
  e2e/lock.e2e.mjs (two browsers); all three browser tests pass against a
  freshly built image.

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

Doku/api/rest.md

@@ -104,7 +104,7 @@ Grundlage: [plans/kollaboration-notizen.md](../plans/kollaboration-notizen.md).
 }
 ```
 
-- Operationsarten: `insert`, `set_content`, `set_props` (`patch`: `type`, `checked`, `collapsed`, `lang`), `move`, `delete`, `restore` (`ids`), `set_title` (`baseVersion`, `title`).
+- Operationsarten: `insert`, `set_content`, `set_props` (`patch`: `type`, `checked`, `collapsed`, `lang`, `clearConflict`), `move`, `delete`, `restore` (`ids`), `set_title` (`baseVersion`, `title`).
 - `leases.hold` ist die vollständige Liste der Blöcke, die der Client halten will; alle anderen Leases des Clients werden freigegeben. Neu eingefügte Blöcke und Konfliktkopien bekommt der Autor automatisch.
 - Höchstens 500 Operationen pro Anfrage. Leser dürfen keine Operationen senden (`403`), aber abfragen.