fix(offline): address PR #120 review (cross-user leak, Yjs title dup, i18n, docs, guards)

Security:
- Clear the offline IndexedDB cache on sign-in (not only logout) so a previous
  user's persisted query cache and Yjs page bodies cannot leak to the next user
  on a shared device when the prior session ended without an explicit logout.

Regressions:
- Remove the double Yjs title write from the AI title-generation path: the title
  editor is bound to the Yjs `title` fragment and the server REST update reseeds
  it, so the local setContent raced that reseed and doubled/garbled the title.

Conventions / i18n / docs:
- Remove the unused showAiMenuAtom.
- Register the 3 offline-fallback strings in en-US and ru-RU.
- Fix the 5 broken links to the nonexistent docs/offline-sync-plan.md.

Stability / simplification:
- warmInfiniteAll now reports truncation (returns false) when it hits maxPages
  with a cursor still pending instead of silently succeeding.
- space-tree make-offline catch logs the raw error and surfaces the real cause.
- Move the Offline/Mobile/CORS CHANGELOG entries from the released 0.93.0 section
  into [Unreleased] (CORS is a documented breaking change).
- Drop the pass-through sync-flag forwarders in use-page-collab-providers; set the
  atoms directly.
- Collapse the three isSwaggerEnabled true-cases into it.each.

Tests / architecture:
- Extract collabTokenNeedsRefresh (pure) and cover all four token states.
- Extract shouldPropagateTitleChange and cover the collab-origin skip; add a
  TitleEditor render test for the static-h1 vs collaborative-editor switch.
- Add a use-auth test asserting the sign-in cache purge runs before login.
- Add an OFFLINE_PERSIST_ROOTS guard test asserting every persisted root maps to
  an exported query-key factory; route make-offline's currentUser warm through a
  new userKeys factory.

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

docs/mobile-bootstrap.md

@@ -20,8 +20,8 @@ mobile app for Gitmost, per the first-step checklist in
   as `NetworkOnly` — offline reads are served by the persisted TanStack Query
   cache (IndexedDB) and `y-indexeddb` for the page Yjs doc, not by an SW HTTP
   cache. This lets the existing responsive web UI be installed and run as a
-  Progressive Web App. See [docs/offline-sync-plan.md](./offline-sync-plan.md) for
-  the full offline/sync design.
+  Progressive Web App. The offline/sync design (stages M0…M4) is summarized in
+  [mobile-app-plan.md](./mobile-app-plan.md).
 - **Backend mobile auth**: opt-in token return from the login flow. The login
   request accepts a `returnToken` flag (must be sent as a JSON boolean) that makes
   the server include the auth token in the response body, and the server already