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>

frontend/src/lib/docSync/__tests__/engine.test.ts

@@ -1,5 +1,5 @@
 import 'fake-indexeddb/auto';
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
 import type { DocumentBlock } from '../../types';
 import { DocSyncEngine, formBatch } from '../engine';
 import { applyOp, cloneDoc, Doc, emptyDoc, Op, Result } from '../model';
@@ -254,6 +254,33 @@ describe('DocSyncEngine', () => {
     expect(serverContents(server)).toEqual(['Start']);
   });
 
+  it('polls every 3 s right after reopening a shared note from local storage', async () => {
+    vi.useFakeTimers();
+    try {
+      const server = new FakeServer();
+      const net = new FakeNet(server);
+      let calls = 0;
+      const sync = net.sync.bind(net);
+      net.sync = async (...args: Parameters<typeof sync>) => {
+        calls++;
+        return sync(...args);
+      };
+      // Opened before: the note is reopened from local storage (like after a reload)
+      const storage = new MemoryStorage();
+      const first = new DocSyncEngine('doc', { transport: net, storage, manual: true });
+      await first.start();
+      first.stop();
+      const engine = new DocSyncEngine('doc', { transport: net, storage });
+      await engine.start();
+      const afterStart = calls;
+      await vi.advanceTimersByTimeAsync(3_500);
+      expect(calls).toBeGreaterThan(afterStart); // not only after 30 s
+      engine.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
   it('formBatch keeps the last text per block with the base of the first', () => {
     const batch = formBatch([
       { opId: '1', kind: 'set_content', blockId: 'a', content: 'x', baseVersion: 5 },