refactor(10-12): migrate sidebar + layout + TreeItem SVGs to AppIcon

- StorageBrowser.vue: replace 8 inline SVGs (plus, folder x2, pencil, trash, share, folderMove, document)
- AppSidebar.vue: replace 8 inline SVGs (tag, inbox, chevronRight x2, folder, cloud, shield, cog dual-path, logout)
- AdminSidebar.vue: replace 6 inline SVGs (home, users, chartBar, lightBulb, clipboardList, logout)
- TreeItem.vue: replace 1 inline SVG (chevronRight expand/collapse)

frontend/src/components/ui/TreeItem.vue

@@ -12,15 +12,11 @@
         class="w-5 h-5 flex items-center justify-center text-gray-400 hover:text-gray-600 shrink-0 transition-colors"
         :aria-label="expanded ? 'Collapse' : 'Expand'"
       >
-        <svg
+        <AppIcon
+          name="chevronRight"
           class="w-3 h-3 transition-transform duration-150"
           :class="{ 'rotate-90': expanded }"
-          fill="none"
-          stroke="currentColor"
-          viewBox="0 0 24 24"
-        >
-          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2.5" d="M9 5l7 7-7 7" />
-        </svg>
+        />
       </button>
       <span v-else class="w-5 h-5 shrink-0"></span>
 
@@ -77,6 +73,7 @@
 
 <script setup>
 import { ref } from 'vue'
+import AppIcon from './AppIcon.vue'
 
 const props = defineProps({
   label: { type: String, required: true },