debug: add comprehensive logging for image preview issue

Added detailed logging at every step of the image conversion and preview:

ImageMagick Service:
- Log input and output sizes
- Log applied quality settings
- Verify result is not empty before creating blob
- Log created blob size and MIME type
- Fail early if conversion produces empty result

Preview Component:
- Log blob details (size, type, URL)
- Add onError handler to img tag with detailed error info
- Add onLoad handler to confirm successful loading
- Console logs will help identify:
  - Is the blob empty?
  - Is the MIME type correct?
  - Is the object URL valid?
  - What error occurs when loading?

TypeScript Fix:
- Changed result type to `Uint8Array | undefined`
- Allows proper checking before blob creation

This will help diagnose why the preview image appears broken
despite successful conversion.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/converter/ConversionPreview.tsx

@@ -68,6 +68,14 @@ export function ConversionPreview({ job, onDownload }: ConversionPreviewProps) {
 
     const category = job.outputFormat.category;
 
+    // Log blob details for debugging
+    console.log('[Preview] Blob details:', {
+      size: job.result.size,
+      type: job.result.type,
+      previewUrl,
+      outputFormat: job.outputFormat.extension,
+    });
+
     switch (category) {
       case 'image':
         return (
@@ -76,6 +84,17 @@ export function ConversionPreview({ job, onDownload }: ConversionPreviewProps) {
               src={previewUrl}
               alt="Converted image preview"
               className="max-w-full max-h-64 object-contain"
+              onError={(e) => {
+                console.error('[Preview] Image failed to load:', {
+                  src: previewUrl,
+                  blobSize: job.result?.size,
+                  blobType: job.result?.type,
+                  error: e,
+                });
+              }}
+              onLoad={() => {
+                console.log('[Preview] Image loaded successfully');
+              }}
             />
           </div>
         );