Boot-failure seatbelt: no future crash becomes a silent white screen

Per Codex. A branded recovery card in app.html shows if the app hasn't mounted
in 7s, or on a pre-mount JS error/unhandledrejection — with a "Refresh Upbeat
Bytes" button. A chunk/preload failure (vite:preloadError) reloads once
(sessionStorage-guarded). +layout calls window.__ubBooted() on mount to clear
the card + timer. A pre-mount failure also fires a tiny anonymous client_error
beacon; the admin Overview now shows "Load errors today" (red if >0) so we can
see if blank-risk is happening in the wild.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -1,8 +1,12 @@
 <script>
   import '../app.css';
+  import { onMount } from 'svelte';
   import FeedbackModal from '$lib/components/FeedbackModal.svelte';
   import { fb, openFeedback, closeFeedback } from '$lib/feedback.svelte.js';
   let { children } = $props();
+  // Tell the boot-failure seatbelt (app.html) the app mounted — clears the
+  // recovery card + timeout as soon as the shell hydrates.
+  onMount(() => window.__ubBooted?.());
 </script>
 
 {@render children()}