Add proxy downloads and bulk file operations

- Implement proxy download endpoint for better mobile Safari compatibility
- Add bulk selection with checkboxes in file browser
- Add bulk actions bar for downloading/deleting selected files
- Replace direct S3 downloads with server-proxied downloads
- Fix download issues on mobile devices by using proper link handling

components/FilePreviewModal.vue

@@ -204,14 +204,12 @@ const downloadFile = async () => {
   if (!props.file) return;
   
   try {
-    const response = await $fetch('/api/files/download', {
-      params: { fileName: props.file.name },
-    });
+    // Use proxy download endpoint for better mobile compatibility
+    const proxyUrl = `/api/files/proxy-download?fileName=${encodeURIComponent(props.file.name)}`;
     
-    // For mobile Safari, we need to use a different approach
+    // Create a link element
     const link = document.createElement('a');
-    link.href = response.url;
-    link.target = '_blank';
+    link.href = proxyUrl;
     
     // Extract clean filename for download
     let filename = props.file.displayName;
@@ -220,9 +218,14 @@ const downloadFile = async () => {
     }
     
     link.download = filename;
+    link.style.display = 'none';
     document.body.appendChild(link);
     link.click();
-    document.body.removeChild(link);
+    
+    // Clean up
+    setTimeout(() => {
+      document.body.removeChild(link);
+    }, 100);
   } catch (err) {
     console.error('Failed to download file:', err);
   }