Change: Add Image Drag-and-Drop and Paste Functionality

Why

The current image upload system only supports file dialog selection, which is inefficient for users who need to quickly add reference images from web pages, screenshots, or file explorers. Adding drag-and-drop and paste functionality will significantly improve user experience and workflow efficiency.

What Changes

• Enable drag-and-drop of image files directly onto the reference image area
• Add support for pasting images from clipboard (screenshots, copied images)
• Maintain backward compatibility with existing file dialog upload
• Enhance image validation to support multiple input methods
• Improve visual feedback for drag-and-drop operations

Impact

• Affected specs: ui/image-upload capability
• Affected code: image_generator.py (image upload and preview components)
• Breaking changes: None (pure enhancement)
• Performance impact: Minimal (event handling overhead)
• User experience: Significantly improved workflow efficiency