feat(ui): add upload speed indicator in Mbps

frontend/src/components/FileExplorer.tsx

@@ -247,6 +247,28 @@ export default function FileExplorer({ onLogout }: FileExplorerProps) {
   const [sortAsc, setSortAsc] = useState(true);
   const [draggedFile, setDraggedFile] = useState<string | null>(null);
   const [uploads, setUploads] = useState<UploadBatch[]>([]);
+  const uploadsRef = useRef<UploadBatch[]>([]);
+  useEffect(() => {
+    uploadsRef.current = uploads;
+  }, [uploads]);
+
+  const [uploadSpeed, setUploadSpeed] = useState<number>(0);
+  const prevUploadBytesRef = useRef<number>(0);
+
+  useEffect(() => {
+    const timer = setInterval(() => {
+      const currentBytes = uploadsRef.current.reduce((acc, u) => acc + u.uploadedBytes, 0);
+      const diff = currentBytes - prevUploadBytesRef.current;
+      setUploadSpeed(diff > 0 ? diff : 0);
+      prevUploadBytesRef.current = currentBytes;
+    }, 1000);
+    return () => clearInterval(timer);
+  }, []);
+
+  const formatSpeed = (bytesPerSec: number) => {
+    const mbps = (bytesPerSec * 8) / 1000000;
+    return ` (${mbps.toFixed(1)} Mbps)`;
+  };
   const [toasts, setToasts] = useState<Toast[]>([]);
   const [deleteConfirm, setDeleteConfirm] = useState<{ type: 'single' | 'multiple'; file?: FileItem; count?: number } | null>(null);
   const [conflictState, setConflictState] = useState<{
@@ -2066,7 +2088,7 @@ export default function FileExplorer({ onLogout }: FileExplorerProps) {
                 <Upload className="w-4 h-4" style={{ color: 'var(--color-accent)' }} />
                 <span className="text-sm font-medium" style={{ color: 'var(--color-text-primary)' }}>
                   {uploads.filter(u => u.status === 'uploading').length > 0
-                    ? `Uploading ${uploads.filter(u => u.status === 'uploading').length} file${uploads.filter(u => u.status === 'uploading').length > 1 ? 's' : ''}`
+                    ? `Uploading ${uploads.filter(u => u.status === 'uploading').length} file${uploads.filter(u => u.status === 'uploading').length > 1 ? 's' : ''}${formatSpeed(uploadSpeed)}`
                     : 'Uploads complete'}
                 </span>
               </div>