feat: add ErrorBoundary component with onErrorCaptured (BUG-F5)

Catches runtime errors in child components, displays user-friendly
error message with reload button. Wired into App.vue wrapping
router-view. Tests verify error capture and button rendering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.vue

@@ -2,6 +2,7 @@
 import { onMounted } from 'vue'
 import { RouterView } from 'vue-router'
 import NavBar from './components/NavBar.vue'
+import ErrorBoundary from './components/ErrorBoundary.vue'
 import { ensureAudioContext } from './game/audio'
 
 // Unlock AudioContext + SpeechSynthesis on first user interaction (mobile requires gesture)
@@ -21,7 +22,9 @@ onMounted(() => {
     <div class="fixed inset-0 crt-overlay z-40" />
     <NavBar />
     <main class="flex-1 min-h-0 relative z-10 overflow-y-auto pb-14 md:pb-0">
-      <RouterView />
+      <ErrorBoundary>
+        <RouterView />
+      </ErrorBoundary>
     </main>
   </div>
 </template>