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}

ui/src/screens/SettingsHub.svelte

@@ -6,6 +6,7 @@
   import Friends from './Friends.svelte';
   import About from './About.svelte';
   import { app } from '../lib/app.svelte';
+  import { offlineMode } from '../lib/offline.svelte';
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
 
   // The Settings hub: a single nav bar + bottom tab bar hosting the Settings / Profile /
@@ -23,6 +24,12 @@
   $effect(() => {
     if (guest && tab === 'friends') tab = 'settings';
   });
+  // Offline mode has no network, so the Profile and Friends surfaces (which read/save over the
+  // network) are disabled — fall back to Settings if the hub is on one (a deep-link or a live flip).
+  // Settings stays reachable: it holds the online/offline toggle.
+  $effect(() => {
+    if (offlineMode.active && (tab === 'profile' || tab === 'friends')) tab = 'settings';
+  });
 
   const titleKey: Record<SettingsTab, MessageKey> = {
     settings: 'settings.title',
@@ -48,11 +55,11 @@
       <button class="tab" class:active={tab === 'settings'} onclick={() => (tab = 'settings')}>
         <span class="face"><span class="sq" aria-hidden="true">⚙️</span><span class="lbl">{t('settings.title')}</span></span>
       </button>
-      <button class="tab" class:active={tab === 'profile'} onclick={() => (tab = 'profile')}>
+      <button class="tab" class:active={tab === 'profile'} disabled={offlineMode.active} onclick={() => (tab = 'profile')}>
         <span class="face"><span class="sq" aria-hidden="true">👤</span><span class="lbl">{t('profile.title')}</span></span>
       </button>
       {#if !guest}
-        <button class="tab" class:active={tab === 'friends'} onclick={() => (tab = 'friends')}>
+        <button class="tab" class:active={tab === 'friends'} disabled={offlineMode.active} onclick={() => (tab = 'friends')}>
           <span class="face"><span class="sq" aria-hidden="true">🤝{#if app.notifications > 0}<span class="badge">{app.notifications}</span>{/if}</span><span class="lbl">{t('friends.title')}</span></span>
         </button>
       {/if}