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/index.html

@@ -143,6 +143,34 @@
           var r = e && e.reason;
           logErr('unhandledrejection', r && (r.stack || r.message) ? r.stack || r.message : String(r));
         });
+        // Mirror console.error / console.warn into the panel. The app catches its boot failures and
+        // surfaces them only as a toast (they never reach window.onerror); a temporary console.error
+        // at the app's handleError routes the real cause here, so the panel shows what actually threw.
+        function fmt(a) {
+          if (a && (a.stack || a.message)) return a.stack || a.message;
+          if (typeof a === 'object') {
+            try {
+              return JSON.stringify(a);
+            } catch (e) {
+              return String(a);
+            }
+          }
+          return String(a);
+        }
+        function wrapConsole(orig, label) {
+          return function () {
+            try {
+              var parts = [];
+              for (var i = 0; i < arguments.length; i++) parts.push(fmt(arguments[i]));
+              logErr('console.' + label, parts.join(' '));
+            } catch (e) {}
+            if (orig) return orig.apply(console, arguments);
+          };
+        }
+        if (window.console) {
+          console.error = wrapConsole(console.error, 'error');
+          console.warn = wrapConsole(console.warn, 'warn');
+        }
 
         // ---- environment -----------------------------------------------------------------------
         var ua = nav.userAgent || '(no userAgent)';

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();