feat(ui): support old Android in-app WebViews (es2019 + core-js + engine screen)

Old Android System WebViews (the Telegram/VK in-app browser; a real device with Google
Play auto-updates the WebView, but emulators / no-Play / restricted devices stay frozen)
showed a white screen. Two causes, fixed in order:

- build.target es2022 shipped ?./?? verbatim, which the old engine cannot parse. Lowered
  to es2019 so esbuild down-levels the syntax.
- The es2020+ runtime globals the bundle and its deps call (globalThis, structuredClone,
  Array.at, ...) are missing on those engines. A conditional core-js loader (emitPolyfills
  writes dist/polyfills.js, document.write'd by an index.html gate only when needed) covers
  them; modern engines download nothing, so the bundle-size budget is untouched.

BigInt (the 64-bit FlatBuffers timestamp decode) and Proxy (Svelte 5 runes) cannot be
polyfilled, so the effective floor is Chrome 67. Below it, a permanent ES5 boot guard in
index.html shows a friendly "this device's OS or browser can't run the app" screen (with
the web-version link inside a Mini App) and a "Diagnostic information" view with a Copy
button, instead of a white screen. A reactive net raises the same screen on an uncaught
boot error that never signals window.__booted.

Board tile glyphs used container-query units (cqw, Chrome 105+) under .cell font-size:0,
so they collapsed to 0px on Chrome 74 (invisible letters); added vmin fallbacks.

The unsupported-engine telemetry beacon is a separate follow-up PR.

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",