feat(offline): transport kill switch + gate the network-requiring UI

Offline mode was 'fiction': the UI only disabled the Stats tab, but Profile was
reachable and a profile save actually hit the server and reported 'saved'.

Two layers now:
- Transport kill switch (transport.ts): in offline mode every network op — each
  unary RPC (exec), the live stream (subscribe), the dict/metrics fetches and
  the reachability probe — is refused before it leaves the device. Offline
  truly means offline regardless of any UI that slips through. The local
  (device-only) game path never uses this transport, so it is unaffected.
- UI gating: the Profile and Friends tabs (SettingsHub) and the Feedback entry
  (About) are disabled offline, and the hub falls back to Settings (which holds
  the online/offline toggle); the lobby Stats tab was already disabled. In-game
  social/export are already hidden for a vs_ai game.

Online unaffected (offlineMode is off): e2e 196. Offline gating is
contour-verified (the mock e2e cannot enter offline).

ui/src/screens/About.svelte

@@ -2,6 +2,7 @@
   import { t } from '../lib/i18n/index.svelte';
   import { app } from '../lib/app.svelte';
   import { navigate } from '../lib/router.svelte';
+  import { offlineMode } from '../lib/offline.svelte';
   import { aboutContent } from '../lib/aboutContent';
   import { onExternalLinkClick } from '../lib/links';
 
@@ -35,7 +36,7 @@
   <p class="muted">{t('about.version', { v: version })}</p>
 
   {#if !(app.profile?.isGuest ?? true)}
-    <button class="feedback" onclick={() => navigate('/feedback')}>
+    <button class="feedback" disabled={offlineMode.active} onclick={() => navigate('/feedback')}>
       {t('feedback.title')}{#if app.feedbackReplyUnread}<span class="fbadge">1</span>{/if}
     </button>
   {/if}