feat(offline): gate the offline toggle on dictionary readiness

Flipping the Settings toggle to offline now checks that every enabled variant's
dictionary is on the device before entering offline mode: it fetches missing ones
cache-first and waits up to ~5 s (raceOfflineReady + the lazy dict/offlineready),
greying the toggle meanwhile. If they cannot be readied in time it stays online and
shows a 'needs internet' note, while the fetch keeps warming the cache in the
background so a later flip is instant. Leaving offline is never gated.

Prevents entering a half-baked offline mode (no dawg -> cannot create/play a local
game) when the background preload has not finished (poor connection, or an immediate
flip right after install).

- offline.ts: raceOfflineReady (pure, injected sleep; unit-tested red->green)
- dict/offlineready.ts: ensureOfflineDicts (cache-first preloadDicts, lazy chunk)
- offline.svelte.ts: requestOffline + TOGGLE_READY_BUDGET_MS
- Settings.svelte: checking/needsData state, disabled toggle, inline note
- i18n: settings.offlineChecking / settings.offlineNeedsData (en+ru)
- docs: FUNCTIONAL(+_ru) offline story + ARCHITECTURE offline paragraph

ui/src/lib/i18n/en.ts

@@ -217,6 +217,8 @@ export const en = {
   'settings.offlineMode': 'Play mode',
   'settings.online': 'Online',
   'settings.offline': 'Offline',
+  'settings.offlineChecking': 'Loading dictionaries…',
+  'settings.offlineNeedsData': 'Not enough data on the device. Internet access is needed to download it.',
   'offline.preloadWarning': 'Poor internet connection. Some features may be unavailable.',
   'offline.promptTitle': 'No connection. Enable offline mode?',
   'offline.promptYes': 'Enable',