feat(offline): background-preload dictionaries for offline readiness

An installed PWA (standalone web + confirmed email) warms the dictionaries
for the player's enabled variants while online — on lobby entry and on a
variant-preference change — using the per-variant version the profile now
advertises, so a later switch to offline mode already has the data.

- dict/preload.ts: pure preloadDicts (retry + linear backoff, honours the
  session dictionary miss-breaker); node-tested.
- dict/preloadrun.ts: lazy browser orchestration (real getDawg), imported
  dynamically so the loader and move generator stay out of the main bundle.
- offline.svelte.ts: kickDictPreload, gated by the pure, tested
  offlinePreloadEligible, plus the reactive first-lobby preload warning.
- Header shows a poor-connection notice in the ad-banner slot on a
  first-lobby preload failure (offline.preloadWarning, en/ru).
- App-entry bundle budget 112->113 for the irreducible main-side wiring
  (documented in bundle-size.mjs); the heavy parts remain lazy chunks.

Docs: ARCHITECTURE offline-mode + dict-preload mechanism.

ui/src/lib/i18n/en.ts

@@ -217,6 +217,7 @@ export const en = {
   'settings.offlineMode': 'Play mode',
   'settings.online': 'Online',
   'settings.offline': 'Offline',
+  'offline.preloadWarning': 'Poor internet connection. Some features may be unavailable.',
 
   'about.title': 'About',
   'about.tab': 'Info',

ui/src/lib/i18n/ru.ts

@@ -217,6 +217,7 @@ export const ru: Record<MessageKey, string> = {
   'settings.offlineMode': 'Режим игры',
   'settings.online': 'Онлайн',
   'settings.offline': 'Оффлайн',
+  'offline.preloadWarning': 'Плохое соединение с интернет. Некоторые функции могут быть недоступны.',
 
   'about.title': 'О программе',
   'about.tab': 'Инфо',