refactor(10-12): migrate folder/document/upload/sharing SVGs to AppIcon

- FolderRow.vue: folder icon + dots menu button (fill-based → stroke AppIcon)
- FolderTreeItem.vue: folder icon in #icon slot
- FolderDeleteModal.vue: warning icon
- DocumentCard.vue: document, folderMove, share action icons
- DocumentPreviewModal.vue: x close button (spinner SVG kept)
- ShareModal.vue: x close button
- DropZone.vue: upload icon
- UploadProgress.vue: exclamationCircle (error) + checkCircle (done) icons; spinner kept

frontend/src/components/folders/FolderDeleteModal.vue

@@ -14,10 +14,7 @@
       <!-- Warning icon -->
       <div class="flex justify-center">
         <div class="w-10 h-10 bg-red-50 rounded-full flex items-center justify-center">
-          <svg class="w-5 h-5 text-red-500" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
-              d="M12 9v2m0 4h.01M10.29 3.86L1.82 18a2 2 0 001.71 3h16.94a2 2 0 001.71-3L13.71 3.86a2 2 0 00-3.42 0z" />
-          </svg>
+          <AppIcon name="warning" class="w-5 h-5 text-red-500" />
         </div>
       </div>
 
@@ -52,6 +49,8 @@
 </template>
 
 <script setup>
+import AppIcon from '../ui/AppIcon.vue'
+
 const props = defineProps({
   folder: {
     type: Object,