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/model.ts

@@ -33,6 +33,7 @@ export interface PropsPatch {
   checked?: boolean;
   collapsed?: boolean;
   lang?: string;
+  clearConflict?: boolean;
 }
 
 export type OpKind = 'insert' | 'set_content' | 'set_props' | 'move' | 'delete' | 'restore' | 'set_title';
@@ -345,6 +346,7 @@ export function applyOp(d: Doc, op: Op, by = '', conflictId: (opId: string) => s
         ...(op.patch.checked !== undefined ? { checked: op.patch.checked } : {}),
         ...(op.patch.collapsed !== undefined ? { collapsed: op.patch.collapsed } : {}),
         ...(op.patch.lang !== undefined ? { lang: op.patch.lang } : {}),
+        ...(op.patch.clearConflict ? { conflictOf: undefined, conflictAuthor: undefined } : {}),
       });
       if (type !== b.type || !propsEqual(props, b.props)) emit({ kind: 'set_props', blockId: b.id, type, props });
       return done({ status: 'applied' });