feat(offline): background-preload dictionaries for offline readiness

An installed PWA (standalone web + confirmed email) warms the dictionaries
for the player's enabled variants while online — on lobby entry and on a
variant-preference change — using the per-variant version the profile now
advertises, so a later switch to offline mode already has the data.

- dict/preload.ts: pure preloadDicts (retry + linear backoff, honours the
  session dictionary miss-breaker); node-tested.
- dict/preloadrun.ts: lazy browser orchestration (real getDawg), imported
  dynamically so the loader and move generator stay out of the main bundle.
- offline.svelte.ts: kickDictPreload, gated by the pure, tested
  offlinePreloadEligible, plus the reactive first-lobby preload warning.
- Header shows a poor-connection notice in the ad-banner slot on a
  first-lobby preload failure (offline.preloadWarning, en/ru).
- App-entry bundle budget 112->113 for the irreducible main-side wiring
  (documented in bundle-size.mjs); the heavy parts remain lazy chunks.

Docs: ARCHITECTURE offline-mode + dict-preload mechanism.

ui/src/lib/offline.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, beforeEach } from 'vitest';
-import { loadOfflinePref, saveOfflinePref, offlineReady, missingDicts } from './offline';
+import { loadOfflinePref, saveOfflinePref, offlineReady, missingDicts, offlinePreloadEligible } from './offline';
 import type { Variant } from './model';
 
 // A minimal in-memory localStorage for the persistence tests (node has none).
@@ -35,4 +35,14 @@ describe('offline mode helpers', () => {
     const has = (v: Variant): boolean => v === 'scrabble_en';
     expect(missingDicts(['scrabble_en', 'scrabble_ru', 'erudit_ru'], has)).toEqual(['scrabble_ru', 'erudit_ru']);
   });
+
+  it('offlinePreloadEligible requires a standalone PWA with email, online, outside mini-apps', () => {
+    const base = { hasEmail: true, standalone: true, inTelegram: false, inVK: false, online: true };
+    expect(offlinePreloadEligible(base)).toBe(true);
+    expect(offlinePreloadEligible({ ...base, hasEmail: false })).toBe(false);
+    expect(offlinePreloadEligible({ ...base, standalone: false })).toBe(false);
+    expect(offlinePreloadEligible({ ...base, inTelegram: true })).toBe(false);
+    expect(offlinePreloadEligible({ ...base, inVK: true })).toBe(false);
+    expect(offlinePreloadEligible({ ...base, online: false })).toBe(false);
+  });
 });