feat(offline): offline-mode state, Settings toggle + blue chrome (Phase C)

The deliberate offline mode is now visible: an installed web-PWA user with a confirmed
email can switch to offline in Settings, and the header turns blue with an "Offline"
chip. (Network gating, the dict preload, the offline lobby + local-game creation, and the
service-worker precache follow in later Phase C steps.)

- offline.ts / offline.svelte.ts: a sticky, device-scoped reactive offline flag
  (offlineMode), distinct from connection.svelte's transient reachability signal, with the
  pure persistence + readiness helpers (offlineReady / missingDicts) unit-tested.
- Settings.svelte: an Online / Offline toggle, shown only for an installed web PWA with a
  confirmed email (the SW-launch + durable-account preconditions).
- Header.svelte: a blue-tinted nav (color-mix from the accent, so it tracks light/dark and
  a Telegram theme override) + an "Offline" chip while offline; the deliberate mode
  suppresses the transient "Connecting…" indicator.

Behaviour-preserving online (the toggle is hidden and offlineMode is false there; e2e 196
green). App entry stays within its size budget (111.6 / 112 KB gzip).

ui/src/screens/Settings.svelte

@@ -10,8 +10,18 @@
   import type { ThemePref } from '../lib/theme';
   import type { BoardLabelMode } from '../lib/boardlabels';
   import { insideTelegram } from '../lib/telegram';
+  import { insideVK } from '../lib/vk';
+  import { offlineMode, setOfflineMode } from '../lib/offline.svelte';
+  import { isStandalone } from '../lib/pwa';
   import InstallApp from '../components/InstallApp.svelte';
 
+  // The offline toggle is for the installed web PWA with a confirmed email only: the service worker
+  // that lets the app launch with no network runs only in a standalone web install (not a mini-app),
+  // and a durable account (email) anchors the device-local games. Elsewhere the control is hidden.
+  const offlineEligible = $derived(
+    isStandalone() && !insideTelegram() && !insideVK() && !!app.profile?.email,
+  );
+
   const themes: ThemePref[] = ['auto', 'light', 'dark'];
   const themeLabel: Record<ThemePref, MessageKey> = {
     auto: 'settings.themeAuto',
@@ -75,6 +85,20 @@
     </label>
   </section>
 
+  {#if offlineEligible}
+    <section>
+      <h3>{t('settings.offlineMode')}</h3>
+      <div class="seg">
+        <button class="opt" class:active={!offlineMode.active} onclick={() => setOfflineMode(false)}>
+          {t('settings.online')}
+        </button>
+        <button class="opt" class:active={offlineMode.active} onclick={() => setOfflineMode(true)}>
+          {t('settings.offline')}
+        </button>
+      </div>
+    </section>
+  {/if}
+
   <!-- Web-only install call-to-action at the bottom of Settings (renders nothing unless the app
        is installable here — see components/InstallApp.svelte / lib/pwa). -->
   <InstallApp />