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/dict/offlineready.ts

@@ -0,0 +1,27 @@
+// Browser orchestration for the offline-toggle readiness wait. Lazily imported by
+// offline.svelte.ts's requestOffline so the dict loader/generator it pulls in stays out of the main
+// bundle. It runs the same cache-first preload as the background warmup (preload.ts), but bounded by
+// a UI wait: it tells the toggle whether flipping to offline can succeed right now, and leaves the
+// fetch running on a timeout so a later flip is instant.
+
+import type { Profile } from '../model';
+import { preloadDicts } from './preload';
+import { getDawg, dictLoadingDisabled } from '../dict';
+import { raceOfflineReady } from '../offline';
+
+/**
+ * ensureOfflineDicts fetches the profile's enabled variants' dictionaries cache-first (instant when
+ * already warm, a network fetch otherwise) and reports, within budgetMs, whether every one is
+ * available — the offline toggle's readiness gate. With no enabled variant there is nothing to play
+ * offline, so it is never ready. The fetch is not aborted on a timeout; it keeps warming the
+ * on-device cache in the background so a later flip to offline succeeds immediately.
+ */
+export async function ensureOfflineDicts(prof: Profile, budgetMs: number): Promise<boolean> {
+  if (prof.variantPreferences.length === 0) return false;
+  const run = preloadDicts(prof.dictVersions, prof.variantPreferences, {
+    getDawg,
+    disabled: dictLoadingDisabled,
+    retries: 1,
+  });
+  return raceOfflineReady(run, budgetMs);
+}