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>

apps/client/src/features/page/tree/components/space-tree-node-menu.tsx

@@ -104,11 +104,16 @@ export function NodeMenu({ node, canEdit }: NodeMenuProps) {
           color: "red",
         });
       }
-    } catch {
+    } catch (err) {
       // makePageAvailableOffline no longer throws, but warmPageYdoc and other
-      // unexpected failures stay guarded here.
+      // unexpected failures stay guarded here. Log the raw error and surface the
+      // real cause to the user instead of a bare generic string (AGENTS.md).
+      console.error("handleMakeAvailableOffline failed", err);
+      const reason =
+        (err as { response?: { data?: { message?: string } } })?.response?.data
+          ?.message ?? (err instanceof Error ? err.message : String(err));
       notifications.show({
-        message: t("Failed to make page available offline"),
+        message: `${t("Failed to make page available offline")}: ${reason}`,
         color: "red",
       });
     }