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>

internal/collab/ops_test.go

@@ -261,3 +261,14 @@ func TestNotifier(t *testing.T) {
 		t.Fatal("Notify must wake waiters")
 	}
 }
+
+func TestClearConflictMarking(t *testing.T) {
+	d := docWith(t, [3]string{"a", "", "Start"})
+	Apply(d, Op{OpID: "1", Kind: OpSetContent, BlockID: "a", BaseVersion: 1, Content: str("A")}, anna, Env{})
+	_, res := Apply(d, Op{OpID: "2", Kind: OpSetContent, BlockID: "a", BaseVersion: 1, Content: str("B")}, ben, Env{})
+	yes := true
+	Apply(d, Op{OpID: "3", Kind: OpSetProps, BlockID: res.ConflictBlockID, Patch: &PropsPatch{ClearConflict: &yes}}, ben, Env{})
+	if p := d.Blocks[res.ConflictBlockID].Props; p.ConflictOf != "" || p.ConflictAuthor != "" {
+		t.Fatalf("conflict marking not cleared: %+v", p)
+	}
+}