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/offline.ts

@@ -42,6 +42,23 @@ export function missingDicts(enabled: readonly Variant[], hasDict: (v: Variant)
   return enabled.filter((v) => !hasDict(v));
 }
 
+/**
+ * raceOfflineReady runs the dictionary fetch `run` against a `budgetMs` wait and reports whether
+ * offline mode can be entered now: ready only when the fetch resolves with nothing still failed
+ * before the budget elapses. On a timeout the caller stops waiting but does NOT abort `run` — it
+ * keeps warming the on-device cache so a later flip to offline is instant. The sleep is injected so
+ * the logic stays pure and unit-tests in the node env.
+ */
+export async function raceOfflineReady(
+  run: Promise<{ failed: readonly unknown[] }>,
+  budgetMs: number,
+  sleep: (ms: number) => Promise<void> = (ms) => new Promise((r) => setTimeout(r, ms)),
+): Promise<boolean> {
+  const elapsed = sleep(budgetMs).then(() => null);
+  const res = await Promise.race([run, elapsed]);
+  return res !== null && res.failed.length === 0;
+}
+
 /**
  * offlinePreloadEligible reports whether a background dictionary preload should run in this
  * context: an installed standalone web PWA (not a Telegram/VK mini-app, not a plain browser tab)