fix(offline): stop offline white-screen and replay paused structural mutations

Fixes the offline-sync defects QA found on PR #120 (#237/#238/#220).

Blank-shell / white-screen on offline reload (HIGH):
- auth-query.tsx: the useCollabToken retry predicate read
  `error.response.status` unguarded. Offline the collab-token POST rejects as
  an axios NETWORK error (isAxiosError true, response undefined), so `.status`
  threw an uncaught TypeError in the React Query retryer BEFORE React mounted,
  white-screening every route. Extracted the predicate as `collabTokenRetry`
  and guarded it with optional chaining (`error.response?.status === 404`).
- user-provider.tsx: gated the whole <Layout> on useCurrentUser() and returned
  a bare `<></>` on any error, blanking every authenticated route offline even
  when cached data existed. Now renders the cached app when a (stale) user is
  present and an explicit OfflineFallback when there is no user to fall back on.
- query-persister.ts / make-offline.ts: persist and warm the ['currentUser']
  query so the auth gate can hydrate offline (pinned pages now survive relaunch).

Offline structural create/move/comment silently lost on reload (HIGH):
- offline-mutations.ts: register setMutationDefaults (default mutationFns) for
  stable mutation keys and tag useCreatePageMutation / useMovePageMutation /
  useCreateCommentMutation with those keys. A paused mutation dehydrated to
  IndexedDB while offline now has a mutationFn after reload, so
  resumePausedMutations() replays it on reconnect instead of no-op'ing.

Tests (client vitest): collabTokenRetry no longer throws on a no-response
network error; UserProvider renders cached children / the offline fallback (not
a blank fragment) on a network error; a rehydrated paused create/move is
replayable via resumePausedMutations; currentUser persist-root coverage.

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

apps/client/src/features/comment/queries/comment-query.ts

@@ -20,6 +20,7 @@ import { notifications } from "@mantine/notifications";
 import { IPagination } from "@/lib/types.ts";
 import { useTranslation } from "react-i18next";
 import { useEffect, useMemo } from "react";
+import { offlineMutationKeys } from "@/features/offline/offline-mutations";
 
 export const RQ_KEY = (pageId: string) => ["comments", pageId];
 
@@ -60,6 +61,9 @@ export function useCreateCommentMutation() {
   const { t } = useTranslation();
 
   return useMutation<IComment, Error, Partial<IComment>>({
+    // Stable key so a paused comment-create restored from IndexedDB after an
+    // offline reload finds its default mutationFn and is replayed on reconnect.
+    mutationKey: offlineMutationKeys.createComment,
     mutationFn: (data) => createComment(data),
     onSuccess: (newComment) => {
       const cache = queryClient.getQueryData(