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/pnpm-lock.yaml

@@ -36,6 +36,9 @@ importers:
       '@types/node':
         specifier: ^22.10.0
         version: 22.19.19
+      core-js-bundle:
+        specifier: ^3.49.0
+        version: 3.49.0
       svelte:
         specifier: ^5.15.0
         version: 5.56.0
@@ -508,6 +511,9 @@ packages:
     resolution: {integrity: sha512-eYm0QWBtUrBWZWG0d386OGAw16Z995PiOVo2B7bjWSbHedGl5e0ZWaq65kOGgUSNesEIDkB9ISbTg/JK9dhCZA==}
     engines: {node: '>=6'}
 
+  core-js-bundle@3.49.0:
+    resolution: {integrity: sha512-WXc7oOsePN3aKFOJVG5zQdi+h/Jm2W0WIPYvRc4IG3vkNcbC2w6LlSzTmnhOl6N1xmOJEzCSNieX3mwF+3zBGw==}
+
   debug@4.4.3:
     resolution: {integrity: sha512-RGwwWnwQvkVfavKVt22FGLw+xYSdzARwm0ru6DhTVA3umU5hZc28V3kO4stgYryrTlLpuvgI9GiijltAjNbcqA==}
     engines: {node: '>=6.0'}
@@ -1132,6 +1138,8 @@ snapshots:
 
   clsx@2.1.1: {}
 
+  core-js-bundle@3.49.0: {}
+
   debug@4.4.3:
     dependencies:
       ms: 2.1.3