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/sharing/ShareModal.vue

@@ -17,9 +17,7 @@
         aria-label="Close"
         class="absolute top-4 right-4 text-gray-400 hover:text-gray-600 transition-colors"
       >
-        <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12" />
-        </svg>
+        <AppIcon name="x" class="w-5 h-5" />
       </button>
 
       <!-- Title -->
@@ -116,6 +114,7 @@
 
 <script setup>
 import { ref, onMounted } from 'vue'
+import AppIcon from '../ui/AppIcon.vue'
 import { useDocumentsStore } from '../../stores/documents.js'
 
 const props = defineProps({