refactor: Enhance error handling for file metadata fetching and parsing

src/app/download/[file_id]/page.tsx

@@ -100,17 +100,28 @@ export default function DownloadPage() {
 
             try {
                 const metaRes = await fetch(`/api/file/${file_id}?token=${token}`);
-                if (!metaRes.ok) {
-                    const err = await metaRes.json();
-                    throw new Error(err.error || 'Failed to fetch file metadata.');
+                let metadata;
+                try {
+                    metadata = await metaRes.json();
+                } catch (jsonError) {
+                    console.error('Failed to parse metadata response as JSON:', jsonError);
+                    throw new Error('Failed to parse file metadata.');
                 }
-                const metadata = await metaRes.json();
+
+                if (!metaRes.ok) {
+                    throw new Error(metadata.error || 'Failed to fetch file metadata.');
+                }
+                
                 setFilename(metadata.filename);
                 setTotalBytes(metadata.size);
 
                 setDownloadState('downloading');
                 const encryptedParts = new Array(metadata.num_parts);
 
+                if (!Array.isArray(metadata.parts)) {
+                    throw new Error('Invalid metadata: parts is not an array.');
+                }
+
                 const downloadPromises = metadata.parts.map(async (part: MetadataPart) => {
                     const response = await fetchWithRetry(`/api/download-part/${file_id}/${part.part_index}`);
                     const buffer = await response.arrayBuffer();