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

docs/FUNCTIONAL.md

@@ -265,8 +265,11 @@ Only **vs_ai** games are playable — New Game creates a device-local game again
 plays entirely in the browser with no backend. Local games are kept on the device, visible only in
 offline mode, and never sync to the account. So a game can be created and played with no connection,
 the app quietly preloads the dictionaries for the player's enabled variants while still online, and
-(once installed) launches from a precached shell even with no network. The mode is device-scoped and
-sticky across launches.
+(once installed) launches from a precached shell even with no network. Switching **to** offline first
+checks that the enabled variants' dictionaries are on the device — if any are missing it fetches them
+and waits briefly, and if they cannot be readied it stays online with a short *needs internet* note
+(the download keeps running in the background, so a later switch is instant). The mode is
+device-scoped and sticky across launches.
 
 The app also **enters offline mode on its own** when it cannot reach the network. On a cold launch
 with no connection it switches to offline for that session; when the device is online but the gateway