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/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