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.svelte.ts

@@ -40,6 +40,26 @@ export function setOfflineMode(on: boolean, persist = true): void {
   if (persist) saveOfflinePref(on);
 }
 
+/** The toggle-flip readiness wait: entering offline waits at most this long for the enabled
+ *  variants' dictionaries before reverting to online (the fetch then continues in the background). */
+export const TOGGLE_READY_BUDGET_MS = 5000;
+
+/**
+ * requestOffline attempts to enter offline mode from the Settings toggle. It fetches the enabled
+ * variants' dictionaries cache-first and, if every one is available within the readiness budget,
+ * switches to a deliberate (persisted) offline mode and returns true. Otherwise it stays online and
+ * returns false — the caller shows the "needs internet" note — while the fetch keeps warming the
+ * cache so a later flip is instant. The readiness glue and the dict loader are imported dynamically,
+ * so neither is pulled into the main bundle. Returns false with no profile.
+ */
+export async function requestOffline(prof: Profile | null, budgetMs = TOGGLE_READY_BUDGET_MS): Promise<boolean> {
+  if (!prof) return false;
+  const m = await import('./dict/offlineready');
+  const ready = await m.ensureOfflineDicts(prof, budgetMs);
+  if (ready) setOfflineMode(true);
+  return ready;
+}
+
 // The dict-preload warning: true when a first-lobby background preload could not fetch every
 // enabled variant's dictionary (typically a poor connection), so offline mode may be incomplete.
 // The lobby shows a notice in place of the ad banner while it holds.