feat: Implement push notification settings backend and UI
## ✅ Push Notification Settings - Fully Functional
### Backend (PHP)
**PushNotificationHandler Updates:**
- Added `SETTINGS_KEY` constant
- `ensure_default_settings()` - Initialize defaults
- `get_default_settings()` - Return default config
- `get_settings()` - Fetch current settings
- `update_settings()` - Save settings
**Default Settings:**
```php
[
'use_logo' => true,
'use_product_images' => true,
'use_gravatar' => false,
'click_action' => '/wp-admin/admin.php?page=woonoow#/orders',
'require_interaction' => false,
'silent' => false,
]
```
**NotificationsController:**
- `GET /notifications/push/settings` - Fetch settings
- `POST /notifications/push/settings` - Update settings
- Permission-protected endpoints
### Frontend (React)
**ChannelConfig Component:**
- Fetches push settings on open
- Real-time state management
- Connected switches and inputs
- Save mutation with loading state
- Toast notifications for success/error
- Disabled state during save
**Settings Available:**
1. **Branding**
- Use Store Logo
- Use Product Images
- Use Customer Gravatar
2. **Behavior**
- Click Action URL (input)
- Require Interaction
- Silent Notifications
### Features
✅ **Backend Storage** - Settings saved in wp_options
✅ **REST API** - GET and POST endpoints
✅ **Frontend UI** - Full CRUD interface
✅ **State Management** - React Query integration
✅ **Loading States** - Skeleton and button states
✅ **Error Handling** - Toast notifications
✅ **Default Values** - Sensible defaults
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