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/lib/i18n/en.ts

@@ -214,6 +214,9 @@ export const en = {
   'settings.labelsClassic': 'Classic',
   'settings.labelsNone': 'None',
   'settings.reduceMotion': 'Reduce motion',
+  'settings.offlineMode': 'Play mode',
+  'settings.online': 'Online',
+  'settings.offline': 'Offline',
 
   'about.title': 'About',
   'about.tab': 'Info',

ui/src/lib/i18n/ru.ts

@@ -214,6 +214,9 @@ export const ru: Record<MessageKey, string> = {
   'settings.labelsClassic': 'Классика',
   'settings.labelsNone': 'Без текста',
   'settings.reduceMotion': 'Меньше анимаций',
+  'settings.offlineMode': 'Режим игры',
+  'settings.online': 'Онлайн',
+  'settings.offline': 'Оффлайн',
 
   'about.title': 'О программе',
   'about.tab': 'Инфо',