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()

frontend/src/components/layout/QuotaBar.vue

@@ -0,0 +1,95 @@
+<template>
+  <div v-if="!loadFailed" class="px-4 py-3 border-t border-gray-100">
+    <div class="flex items-center justify-between mb-1">
+      <span class="text-sm font-semibold text-gray-500">Storage</span>
+      <span v-if="ready" class="text-sm" :class="labelColor">{{ label }}</span>
+    </div>
+    <!-- Real bar (shown after first successful fetchQuota) -->
+    <div
+      v-if="ready"
+      class="w-full h-2 bg-gray-200 rounded-full overflow-hidden"
+    >
+      <div
+        role="progressbar"
+        :aria-valuenow="pct"
+        aria-valuemin="0"
+        aria-valuemax="100"
+        :aria-label="`Storage usage: ${label}`"
+        class="h-2 rounded-full transition-all duration-500"
+        :class="barColor"
+        :style="{ width: `${pct}%` }"
+      ></div>
+    </div>
+    <!-- Skeleton loading state (before first fetchQuota completes) -->
+    <div
+      v-else
+      class="w-full h-2 bg-gray-100 rounded-full animate-pulse"
+      aria-label="Loading storage usage"
+      aria-busy="true"
+    ></div>
+  </div>
+</template>
+
+<script setup>
+import { ref, computed, onMounted } from 'vue'
+import { useAuthStore } from '../../stores/auth.js'
+
+const authStore = useAuthStore()
+const ready = ref(false)
+const loadFailed = ref(false)
+
+/**
+ * pct — usage percentage clamped to 0–100.
+ * When limit_bytes is 0 (initial state or server returned 0), returns 0 — bar stays at 0%.
+ * UI-SPEC: do not hide widget when used_bytes=0.
+ */
+const pct = computed(() => {
+  const { used_bytes, limit_bytes } = authStore.quota
+  if (!limit_bytes || limit_bytes <= 0) return 0
+  return Math.min(100, Math.round((used_bytes / limit_bytes) * 100))
+})
+
+/**
+ * Bar fill color per UI-SPEC Quota Bar Color Logic:
+ *   < 80%  → bg-indigo-500
+ *   80–95% → bg-amber-500
+ *   ≥ 95%  → bg-red-500
+ */
+const barColor = computed(() => {
+  if (pct.value >= 95) return 'bg-red-500'
+  if (pct.value >= 80) return 'bg-amber-500'
+  return 'bg-indigo-500'
+})
+
+/**
+ * Label text color follows bar color thresholds (UI-SPEC):
+ *   < 80%  → text-gray-500
+ *   80–95% → text-amber-600
+ *   ≥ 95%  → text-red-600
+ */
+const labelColor = computed(() => {
+  if (pct.value >= 95) return 'text-red-600'
+  if (pct.value >= 80) return 'text-amber-600'
+  return 'text-gray-500'
+})
+
+/**
+ * Label format: "{used} MB of {limit} MB" (1 decimal place, UI-SPEC)
+ */
+const label = computed(() => {
+  const used = (authStore.quota.used_bytes / 1048576).toFixed(1)
+  const limit = (authStore.quota.limit_bytes / 1048576).toFixed(1)
+  return `${used} MB of ${limit} MB`
+})
+
+onMounted(async () => {
+  try {
+    await authStore.fetchQuota()
+    ready.value = true
+  } catch {
+    // fetchQuota itself swallows errors; this catch handles any unexpected re-throw.
+    // On any error: hide widget entirely (loadFailed=true → v-if removes DOM, UI-SPEC)
+    loadFailed.value = true
+  }
+})
+</script>