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/package.json

@@ -27,6 +27,7 @@
     "@playwright/test": "^1.49.0",
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@types/node": "^22.10.0",
+    "core-js-bundle": "^3.49.0",
     "svelte": "^5.15.0",
     "svelte-check": "^4.1.0",
     "typescript": "^5.7.0",