feat(ui): unsupported-engine boot screen; drop the temporary diagnostic

Replace the temporary on-device boot diagnostic (the always-on overlay, the
console.error mirror and the handleError console hook) with a permanent boot-capability
guard in index.html. Before the deferred module, in plain ES5, it:
  - hard-gates on the unpolyfillable essentials the app cannot run without — BigInt (the
    64-bit FlatBuffers wire decode) and Proxy (Svelte 5 runes) — and, when one is
    missing, shows a friendly full-screen "this device's OS/browser can't run the app"
    screen instead of a white screen;
  - soft-gates the polyfillable es2020+ globals, pulling core-js (polyfills.js) only
    when needed (the previous behaviour, folded in);
  - keeps a reactive net: an uncaught error during boot with no window.__booted signal
    within 8s raises the same screen with the captured cause.
Inside a Telegram/VK Mini App the screen also points to the web version
(https://<host>/app/). A "Diagnostic information" button reveals the engine, the
feature table, the reason and the app version, with a Copy button.

App.svelte sets window.__booted once bootstrap resolves. vite.config drops the
now-unneeded strip-boot-diag plugin and adds inject-boot-version (stamps the guard's
diagnostic with VITE_APP_VERSION). The es2019 target, the core-js loader and the board
cqw->vmin glyph fallback stay. The speculative Chrome-74 pan change is reverted — it did
not fix the judder and added nothing on modern engines.

Verified: svelte-check clean, unit + mock e2e green (the guard is inert on a capable
engine, so the size budget and the smoke are untouched), and the screen + diagnostic +
stamped version render on a forced hard-gate (BigInt removed via addInitScript).

The unsupported-engine telemetry beacon (dedup + edge rate-limit + a Prometheus counter)
is a separate follow-up PR, per the agreed split.

ui/vite.config.ts

@@ -4,19 +4,17 @@ import { defineConfig, type Plugin } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
 
 /**
- * stripBootDiag removes the temporary on-device boot-diagnostic block from index.html — the span
- * between the `<!-- BOOT-DIAG:START -->` and `<!-- BOOT-DIAG:END -->` markers. That block is a
- * full-screen ES5 overlay meant only for the production build shipped to the test contour, so it
- * is dropped from every non-production build: the `mock` e2e (whose first taps the overlay would
- * intercept) and the dev server. It is wired only when mode !== 'production'. Remove this plugin
- * together with the index.html block once the Android WebView diagnosis is finished — it must
- * never reach master / production.
+ * injectBootVersion stamps the app version into index.html's boot-capability guard, replacing its
+ * __BOOT_VERSION__ placeholder with the same VITE_APP_VERSION build-arg that feeds __APP_VERSION__.
+ * The guard runs before the bundle, so it cannot read the bundle's version; this lets its on-demand
+ * diagnostic report name the client version anyway. Falls back to "dev" for a local build.
  */
-function stripBootDiag(): Plugin {
+function injectBootVersion(): Plugin {
+  const version = process.env.VITE_APP_VERSION || 'dev';
   return {
-    name: 'strip-boot-diag',
+    name: 'inject-boot-version',
     transformIndexHtml(html) {
-      return html.replace(/\s*<!-- BOOT-DIAG:START[\s\S]*?BOOT-DIAG:END -->/g, '');
+      return html.replace(/__BOOT_VERSION__/g, version);
     },
   };
 }
@@ -56,11 +54,9 @@ export default defineConfig(({ mode }) => ({
     // so a missing build-arg never breaks the build.
     __APP_VERSION__: JSON.stringify(process.env.VITE_APP_VERSION || 'dev'),
   },
-  // emitPolyfills always ships dist/polyfills.js (loaded only by old engines; see its docstring
-  // and the index.html gate). The boot-diagnostic overlay ships only in the production build (the
-  // test contour) — it is stripped from the dev server and the mock e2e build, where a full-screen
-  // overlay would intercept Playwright's taps. See stripBootDiag / the index.html BOOT-DIAG block.
-  plugins: [svelte(), emitPolyfills(), ...(mode !== 'production' ? [stripBootDiag()] : [])],
+  // emitPolyfills ships dist/polyfills.js (loaded only by old engines; see its docstring + the
+  // index.html boot guard). injectBootVersion stamps the app version into that guard's diagnostic.
+  plugins: [svelte(), emitPolyfills(), injectBootVersion()],
   server: {
     port: 5173,
     proxy: