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/src/App.svelte

@@ -26,7 +26,12 @@
   import TelegramLaunchError from './screens/TelegramLaunchError.svelte';
 
   onMount(() => {
-    void bootstrap();
+    // Tell the index.html boot guard that startup completed, so its reactive net does not raise the
+    // unsupported-engine screen on a healthy boot — it only fires when an uncaught error occurred
+    // AND this flag never sets (a bundle that failed to parse, or an unforeseen incompatibility).
+    void bootstrap().then(() => {
+      (window as unknown as { __booted?: boolean }).__booted = true;
+    });
   });
 
   // The lobby is the cold-start landing (an empty hash and Telegram launch params both parse