feat(03-05): create QuotaBar.vue; embed in AppSidebar between topics nav and footer

- QuotaBar.vue: new sidebar quota widget — onMounted calls authStore.fetchQuota(); computed pct/barColor/labelColor/label from authStore.quota; color thresholds <80% indigo-500, 80-95% amber-500, >=95% red-500; skeleton loading state (animate-pulse); hides on loadFailed (v-if); role=progressbar with aria-valuenow/aria-valuemin/aria-valuemax/aria-label
- AppSidebar.vue: import QuotaBar; insert <QuotaBar /> between </nav> and settings footer div
- Frontend build exits 0 (verified)

frontend/src/components/layout/AppSidebar.vue

@@ -54,6 +54,9 @@
       </div>
     </nav>
 
+    <!-- Quota bar (between topics nav and settings footer, UI-SPEC Phase 3) -->
+    <QuotaBar />
+
     <!-- Settings + Admin link -->
     <div class="px-3 py-4 border-t border-gray-100">
       <!-- Admin link (admin users only) -->
@@ -108,6 +111,7 @@
 import { useRouter } from 'vue-router'
 import { useTopicsStore } from '../../stores/topics.js'
 import { useAuthStore } from '../../stores/auth.js'
+import QuotaBar from './QuotaBar.vue'
 
 const topicsStore = useTopicsStore()
 const authStore = useAuthStore()