Loading Skeleton Screens

Create loading skeleton components with shimmer animation for:

1. Card skeleton: matches a product card layout with image area, title lines, and button
2. Table skeleton: 5 rows x 4 columns with varied width skeleton lines
3. Profile skeleton: circular avatar, name line, bio lines, 3 stat boxes
4. List item skeleton: avatar circle + two text lines

All skeletons:
- Shimmer animation sweeping left to right
- Light gray base with lighter shimmer (light mode), darker in dark mode
- Match real component dimensions exactly
- Toggle to switch between skeleton and loaded state for demo
- Use Tailwind CSS, shadcn/ui Skeleton component