fix(offline): address PR #120 review (comment 2513)

CHANGELOG: stop presenting the service-worker API cache as an active offline
store (/api is NetworkOnly) — describe it as a defensive purge of the legacy
api-get-cache from older clients; add an explicit upgrade note that the new CORS
allowlist rejects previously-allowed cross-domain REST clients until their origin
is added to CORS_ALLOWED_ORIGINS.

test(offline): cover make-offline ancestor-walk + dedup — a real-ancestor case
exercising the ancestorId===pageId guard (page warmed once), the dedup of
repeated tree failures into a single "tree" label, and the "breadcrumbs" label
when the breadcrumbs lookup rejects.

test(auth): cover clearOfflineCache in handleLogout — purged exactly once before
window.location.replace, and a thrown purge error does not block the redirect.

conventions: use pageKeys.detail() instead of raw ["pages", …] literals in
title-editor and use-page-collab-providers.

cleanup: remove the dead emit() in title-editor (the gateway ignores it; the
cross-user tree refresh is server-side via the Yjs title fragment); drop the
trivial Array.isArray(tiptapExtensions) test (schema is exercised transitively).

refactor: extract the shared page.<id> Yjs doc-name convention into
pageYdocName()/PAGE_YDOC_NAME_PREFIX so the editor providers, offline warm, and
offline purge can no longer drift apart.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/collaboration/collaboration.util.spec.ts

@@ -7,7 +7,6 @@ import {
   prosemirrorNodeToYElement,
   buildTitleSeedYdoc,
   jsonToText,
-  tiptapExtensions,
 } from './collaboration.util';
 import { Node } from '@tiptap/pm/model';
 
@@ -284,10 +283,4 @@ describe('buildTitleSeedYdoc', () => {
     expect(text1).toBe(title);
     expect(text2).toBe(text1);
   });
-
-  // Touch tiptapExtensions so the import is exercised (mirrors the brief's import
-  // list and guards against accidental tree-shaking of the schema dependency).
-  it('uses the shared tiptap extensions schema', () => {
-    expect(Array.isArray(tiptapExtensions)).toBe(true);
-  });
 });