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/scripts/bundle-size.mjs

@@ -20,10 +20,12 @@ import { join } from 'node:path';
 const DIST = 'dist';
 
 // Per-chunk gzip budgets in KB. The app entry was raised to 110 for the local move-preview
-// wiring, then to 112 for the PWA install feature (the install CTA + the pwa detection /
-// service-worker registration live in the app entry; the heavy dict subsystem stays in lazy
-// chunks). Its scoped CSS lands in the CSS chunk, not this JS budget.
-const BUDGET = { app: 112, shared: 30, landing: 5 };
+// wiring, to 112 for the PWA install feature, then to 113 for the offline-mode wiring: the
+// dictionary-preload trigger and the offline-state reactives live in the app entry (the Header
+// reads them, the lobby/profile screens fire the trigger), while the heavy parts — the dict
+// loader, the move generator and the preload orchestration — stay in lazy chunks. Scoped CSS
+// lands in the CSS chunk, not this JS budget.
+const BUDGET = { app: 113, shared: 30, landing: 5 };
 
 // gzipped returns the gzipped byte size of a built asset, or 0 when the reference is not a
 // local file (e.g. the Telegram SDK loaded from a CDN) or is missing.