feat(offline): PWA shell, Yjs-backed titles, and offline read cache (M0–M2)

Implements docs/offline-sync-plan.md milestones M0–M2.

M0 (PWA shell):
- Add vite-plugin-pwa (generateSW, registerType: 'prompt', manifest:false);
  NetworkOnly for /api,/collab,/socket.io, NetworkFirst for GET /api,
  navigateFallback to index.html.
- Register SW via useRegisterSW with a Mantine update prompt; skip
  registration inside Capacitor native WebView (is-capacitor guard).

M1 (harden CRDT body + title into Yjs):
- Lift the per-page Y.Doc/Hocuspocus providers into a shared hook+context so
  body and title editors share one doc.
- Move the page title into a dedicated 'title' Yjs fragment (CRDT, offline-
  tolerant); drop the REST title save. Server persists the title fragment to
  page.title and seeds it for legacy pages (empty-fragment guard); a collab
  rename emits a treeUpdate so other users' tree/breadcrumbs refresh.
- Persist the rebuilt ydoc on the content->ydoc path to neutralize the Yjs
  duplication trap. Add a 3-state sync indicator.

M2 (offline read/navigation):
- Persist React Query to IndexedDB (idb-keyval persister, version buster,
  selected roots only).
- "Make available offline" action warms page, space, tree (root+ancestors+
  children) and comments under exact hook keys, plus the page ydoc.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -59,6 +59,7 @@ import { generateHTML, generateJSON } from '../common/helpers/prosemirror/html';
 import { Node, Schema } from '@tiptap/pm/model';
 import * as Y from 'yjs';
 import { Logger } from '@nestjs/common';
+import { TiptapTransformer } from '@hocuspocus/transformer';
 
 export const tiptapExtensions = [
   StarterKit.configure({
@@ -143,6 +144,34 @@ export function jsonToText(tiptapJson: JSONContent) {
   return generateText(tiptapJson, tiptapExtensions);
 }
 
+/**
+ * Build a standalone Y.Doc that holds ONLY the page title, in a dedicated Yjs
+ * fragment named exactly 'title' (the collaborative title-editor contract with
+ * the client). The ProseMirror shape is a doc with a single level-1 heading
+ * whose text is the title (empty title => heading with no text child).
+ *
+ * The encoded state of the returned doc can be merged into a body doc via
+ * `Y.applyUpdate(doc, Y.encodeStateAsUpdate(titleSeed))` to seed the title
+ * fragment for legacy pages. Seeding MUST be guarded by an emptiness check on
+ * the existing 'title' fragment to avoid the Yjs duplication trap.
+ */
+export function buildTitleSeedYdoc(title: string): Y.Doc {
+  return TiptapTransformer.toYdoc(
+    {
+      type: 'doc',
+      content: [
+        {
+          type: 'heading',
+          attrs: { level: 1 },
+          content: title ? [{ type: 'text', text: title }] : [],
+        },
+      ],
+    },
+    'title',
+    tiptapExtensions,
+  );
+}
+
 export function jsonToNode(tiptapJson: JSONContent) {
   const schema = getSchema(tiptapExtensions);
   try {