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/ARCHITECTURE.md

@@ -1259,7 +1259,11 @@ eligible installed PWA (standalone web + confirmed email) **background-preloads*
 — on lobby entry and on a variant-preference change — through the same three-tier loader, retried
 with backoff and honouring the session miss-breaker; the move generator, the loader and the preload
 orchestration stay in lazy chunks. A first-lobby preload failure shows a *poor-connection* notice in
-the ad-banner slot. A **cold launch already in offline mode** boots from the persisted session and
+the ad-banner slot. Flipping the Settings toggle **to** offline runs that same cache-first fetch
+bounded by a ~5 s UI wait (`raceOfflineReady` + the lazy `dict/offlineready`): it enters offline only
+once every enabled variant is ready, otherwise it stays online with a *needs internet* note while the
+fetch finishes in the background (the next flip is then instant). A **cold launch already in offline
+mode** boots from the persisted session and
 profile (the profile is saved on every online adopt/refresh) — `bootstrap` skips the session
 adoption and profile fetch that would otherwise hang with no network, and lands straight in the
 offline lobby; without a cached profile (an install that was never online) it drops the sticky flag