TakePOS Multi-screen v2

Changes:
- Proper package root: takepos/
- Right-side embedded customer preview in cashier screen
- Popup customer screen button improved
- No extra custom permissions added (prevents duplicated permission rows)
- Uses existing TakePOS run permission only

Use:
1. Replace takepos folder
2. Open POS
3. Right-side preview appears on wide screens
4. Click Customer Screen to open second fullscreen window
