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/features/workspace/__tests__/OutlinerBlock.caret.test.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import { describe, it, expect, afterEach } from 'vitest';
+import { describe, it, expect, afterEach, vi } from 'vitest';
 import { render, fireEvent } from '@testing-library/react';
 import { OutlinerBlock, mapRenderedOffsetToRaw } from '../OutlinerBlock';
 import { DocumentBlock } from '../../../lib/types';
@@ -62,3 +62,59 @@ describe('clicking into an unfocused bullet', () => {
     expect(ta.selectionStart).toBe(10); // "Hallo **We|lt** du"
   });
 });
+
+function renderBlock(block: DocumentBlock, extra: Record<string, unknown> = {}) {
+  return render(
+    <OutlinerBlock
+      docId="d"
+      block={block}
+      index={0}
+      isLast
+      maxAllowedLevel={1}
+      hasChildren={false}
+      isFocused={false}
+      canEdit
+      onUpdateBlock={() => {}}
+      onCreateBlockBelow={() => {}}
+      onDeleteBlock={() => {}}
+      onIndentBlock={() => {}}
+      onNavigateFocus={() => {}}
+      registerTextareaRef={() => {}}
+      {...extra}
+    />
+  );
+}
+
+describe('undo shortcut', () => {
+  it('saves pending typing first, then undoes', () => {
+    const calls: string[] = [];
+    const block = { id: 'b1', type: 'bullet', content: 'Alt', level: 0 } as DocumentBlock;
+    const { container } = renderBlock(block, {
+      onUpdateBlock: (b: DocumentBlock) => calls.push(`update:${b.content}`),
+      onUndo: () => calls.push('undo'),
+      onRedo: () => calls.push('redo'),
+    });
+    const ta = container.querySelector('textarea') as HTMLTextAreaElement;
+    ta.focus();
+    fireEvent.change(ta, { target: { value: 'Alt neu' } });
+    fireEvent.keyDown(ta, { key: 'z', ctrlKey: true });
+    fireEvent.keyDown(ta, { key: 'z', ctrlKey: true, shiftKey: true });
+    fireEvent.keyDown(ta, { key: 'y', metaKey: true });
+    expect(calls).toEqual(['update:Alt neu', 'undo', 'redo', 'redo']);
+  });
+});
+
+describe('conflict copies', () => {
+  it('show who lost and offer accept/discard', () => {
+    const onResolve = vi.fn();
+    const block = { id: 'cf', type: 'bullet', content: 'Meine Version', level: 0, conflictOf: 'b1', conflictAuthor: 'Ben' } as DocumentBlock;
+    const { getByText } = renderBlock(block, { onResolveConflict: onResolve });
+    expect(getByText(/Version von Ben/)).toBeTruthy();
+    fireEvent.click(getByText('Übernehmen'));
+    fireEvent.click(getByText('Verwerfen'));
+    expect(onResolve.mock.calls).toEqual([
+      ['cf', true],
+      ['cf', false],
+    ]);
+  });
+});