chore(ui): surface caught boot errors on the diagnostic panel

The es2019 + conditional core-js fixes let the module mount on the Chromium 66 in-app
WebView, but the lobby then shows a white screen and a generic "something went wrong"
toast — a caught error that never reaches window.onerror, so the diagnostic ERRORS
panel stayed empty and could not name the cause.

Mirror console.error / console.warn into the panel, and add a temporary, mock-gated
console.error of the raw error (with stack) in handleError, so the on-device panel
shows exactly what threw and whether it is one error or several. Expected: a
ReferenceError from the 64-bit FlatBuffers timestamp decode (BigInt, absent on Chrome
66 and not polyfillable by core-js) — to be confirmed on-device before deciding the
support floor.

Temporary — both hunks revert together with the index.html BOOT-DIAG block.

ui/src/lib/app.svelte.ts

@@ -283,6 +283,12 @@ export function markChatRead(gameId: string): void {
  *  failure — or anything raised while the app is mid-reconnect — is shown by the "Connecting…"
  *  header indicator (and auto-retried), never a red toast. */
 export function handleError(err: unknown): void {
+  // TEMP (boot diagnostic): the toast is generic, so route the real caught error to the on-device
+  // BOOT-DIAG panel via console (it mirrors console.error). Mode-gated to keep it out of the mock
+  // e2e. Revert together with the index.html BOOT-DIAG block.
+  if (import.meta.env.MODE !== 'mock') {
+    console.error('[diag] handleError:', err instanceof Error ? err.stack || err.message : err);
+  }
   const code = err instanceof GatewayError ? err.code : '';
   if (code === 'session_invalid' || code === 'unauthenticated') {
     void logout();