fix(ui): conditionally load core-js polyfills for old Android WebViews

The es2019 down-level fixed the parse error, but the Chromium 66 in-app WebView
(Android 9) then threw "Uncaught ReferenceError: globalThis is not defined" — esbuild
lowers syntax, not the es2020+ runtime globals the bundle and its deps reference
(globalThis, structuredClone, Array.at, Object.hasOwn, Promise.allSettled, …), so the
module still failed to evaluate and the SPA stayed a white screen.

Load core-js only where it is actually missing. A permanent ES5 gate in index.html
feature-detects the engine and, on an old one, document.writes a parser-blocking
<script src=polyfills.js> before the deferred module runs; a modern engine matches
none of the checks and downloads nothing. document.write is used deliberately — it is
the only way to inject a script guaranteed to run before a deferred module — with a
static literal argument (no interpolation, no injection surface). vite.config
emitPolyfills writes dist/polyfills.js from the prebuilt core-js-bundle and keeps it
out of the module graph, so the bundle-size budget is unaffected (app entry still
108.8 / 110 KB gzip).

pnpm-workspace denies core-js-bundle's install script (the package ships a prebuilt
minified.js we only read at build time; its script is a funding banner). svelte-check
and the mock e2e (Chromium + WebKit) are green and the size gate still passes. The
on-device re-test on the Chromium 66 emulator is pending.

ui/vite.config.ts

@@ -1,3 +1,4 @@
+import { readFileSync } from 'node:fs';
 import { resolve } from 'node:path';
 import { defineConfig, type Plugin } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
@@ -20,6 +21,24 @@ function stripBootDiag(): Plugin {
   };
 }
 
+/**
+ * emitPolyfills writes the prebuilt core-js bundle to `dist/polyfills.js`. The index.html gate
+ * script loads it via document.write only on an old engine that lacks the es2020+ runtime APIs the
+ * app uses (globalThis, structuredClone, Array.at, …) — e.g. the Chromium 66 Android System WebView
+ * behind the Telegram/VK in-app browser — before the deferred module runs. A modern engine never
+ * requests it, so it adds nothing to that payload and stays out of the module graph the bundle-size
+ * gate measures.
+ */
+function emitPolyfills(): Plugin {
+  return {
+    name: 'emit-polyfills',
+    generateBundle() {
+      const src = readFileSync(resolve(import.meta.dirname, 'node_modules/core-js-bundle/minified.js'), 'utf8');
+      this.emitFile({ type: 'asset', fileName: 'polyfills.js', source: src });
+    },
+  };
+}
+
 // The edge Connect service is scrabble.edge.v1.Gateway; the gateway serves it over
 // h2c on :8081 by default. In dev we proxy the RPC path so the browser (which can
 // not speak h2c directly) talks to the dev server on the same origin. In `mock`
@@ -37,10 +56,11 @@ export default defineConfig(({ mode }) => ({
     // so a missing build-arg never breaks the build.
     __APP_VERSION__: JSON.stringify(process.env.VITE_APP_VERSION || 'dev'),
   },
-  // The boot-diagnostic overlay ships only in the production build (the test contour); it is
-  // stripped from the dev server and the mock e2e build, where a full-screen overlay would
-  // intercept Playwright's taps. See stripBootDiag / the index.html BOOT-DIAG block.
-  plugins: [svelte(), ...(mode !== 'production' ? [stripBootDiag()] : [])],
+  // emitPolyfills always ships dist/polyfills.js (loaded only by old engines; see its docstring
+  // and the index.html gate). The boot-diagnostic overlay ships only in the production build (the
+  // test contour) — it is stripped from the dev server and the mock e2e build, where a full-screen
+  // overlay would intercept Playwright's taps. See stripBootDiag / the index.html BOOT-DIAG block.
+  plugins: [svelte(), emitPolyfills(), ...(mode !== 'production' ? [stripBootDiag()] : [])],
   server: {
     port: 5173,
     proxy:
@@ -57,9 +77,10 @@ export default defineConfig(({ mode }) => ({
     // Down-level to es2019 so an old Android System WebView (seen: Telegram/VK in-app WebView on
     // Chromium 66, Android 9) can parse the bundle — esbuild lowers es2020+ syntax (?., ??, private
     // fields, static blocks, …) that Chrome 66 rejects with "Unexpected token ?", which left the SPA
-    // on a white screen. esbuild lowers syntax only, not runtime APIs; the app's own source uses
-    // none beyond this floor (the mock-only helpers are tree-shaken from production). Reach for
-    // @vitejs/plugin-legacy if runtime-API polyfills for even older engines turn out to be needed.
+    // on a white screen. esbuild lowers syntax only, not runtime APIs — the es2020+ globals the
+    // bundle/deps call at runtime (globalThis, structuredClone, Array.at, …) are covered separately
+    // by the conditional core-js loader (emitPolyfills + the index.html gate), loaded only on an
+    // engine that actually lacks them.
     target: 'es2019',
     // Emit sourcemaps everywhere except the production build. A shipped `.map`
     // carries full `sourcesContent` — the entire TypeScript/Svelte source — and the