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/screens/Settings.svelte

@@ -11,7 +11,7 @@
   import type { BoardLabelMode } from '../lib/boardlabels';
   import { insideTelegram } from '../lib/telegram';
   import { insideVK } from '../lib/vk';
-  import { offlineMode, setOfflineMode } from '../lib/offline.svelte';
+  import { offlineMode, setOfflineMode, requestOffline } from '../lib/offline.svelte';
   import { isStandalone } from '../lib/pwa';
   import InstallApp from '../components/InstallApp.svelte';
 
@@ -35,6 +35,25 @@
     classic: 'settings.labelsClassic',
     none: 'settings.labelsNone',
   };
+
+  // The offline toggle gates entry on dictionary readiness: flipping to offline fetches the enabled
+  // variants' dictionaries (cache-first) and waits at most a few seconds; if they cannot be made
+  // available it stays online and shows the "needs internet" note, while the fetch keeps warming the
+  // cache in the background so a later flip is instant. Leaving offline (-> online) is never gated.
+  let checking = $state(false);
+  let needsData = $state(false);
+
+  async function goOffline(): Promise<void> {
+    if (offlineMode.active || checking) return;
+    needsData = false;
+    checking = true;
+    try {
+      const ready = await requestOffline(app.profile);
+      if (!ready) needsData = true;
+    } finally {
+      checking = false;
+    }
+  }
 </script>
 
 <div class="page">
@@ -89,13 +108,26 @@
     <section>
       <h3>{t('settings.offlineMode')}</h3>
       <div class="seg">
-        <button class="opt" class:active={!offlineMode.active} onclick={() => setOfflineMode(false)}>
+        <button
+          class="opt"
+          class:active={!offlineMode.active}
+          disabled={checking}
+          onclick={() => {
+            needsData = false;
+            setOfflineMode(false);
+          }}
+        >
           {t('settings.online')}
         </button>
-        <button class="opt" class:active={offlineMode.active} onclick={() => setOfflineMode(true)}>
+        <button class="opt" class:active={offlineMode.active} disabled={checking} onclick={goOffline}>
           {t('settings.offline')}
         </button>
       </div>
+      {#if checking}
+        <p class="onote">{t('settings.offlineChecking')}</p>
+      {:else if needsData}
+        <p class="onote onote--warn" role="alert">{t('settings.offlineNeedsData')}</p>
+      {/if}
     </section>
   {/if}
 
@@ -139,6 +171,18 @@
     color: var(--accent-text);
     border-color: var(--accent);
   }
+  .opt:disabled {
+    opacity: 0.55;
+    cursor: default;
+  }
+  .onote {
+    font-size: 0.85rem;
+    color: var(--text-muted);
+    margin: 8px 0 0;
+  }
+  .onote--warn {
+    color: var(--warn);
+  }
   .row {
     display: flex;
     align-items: center;