refactor: redesign debug panel with overlay visualization

Much better UX for identifying correct rotation:

- Shows ORIGINAL image scaled to fit the modal
- Draws GREEN bounding box showing the crop area
- Draws ORANGE rotated rectangle showing rotation effect
- Darkens everything outside the crop area (visual focus)
- Rotation slider updates the overlay in real-time
- Quick preset buttons for common angles
- Detailed debug logs showing all calculations
- Better styling: dark theme for readability

User can now visually see what will be extracted with the current
rotation angle. Report the angle that makes the label readable.

frontend/components/ItemDetailModal.tsx

@@ -201,6 +201,7 @@ export default function ItemDetailModal({
           originalCropBounds={item.labels_data ? JSON.parse(item.labels_data).image_processing?.crop_bounds : undefined}
           originalRotation={item.labels_data ? JSON.parse(item.labels_data).image_processing?.rotation_degrees : undefined}
           imageProcessing={item.labels_data ? JSON.parse(item.labels_data).image_processing : undefined}
+          onClose={() => setShowDebugPanel(false)}
         />
       )}
     </div>