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

---

**Next: Email channel toggle** 📧

includes/Api/NotificationsController.php

@@ -120,6 +120,24 @@ class NotificationsController {
 				'permission_callback' => '__return_true',
 			],
 		]);
+		
+		// GET /woonoow/v1/notifications/push/settings
+		register_rest_route($this->namespace, '/' . $this->rest_base . '/push/settings', [
+			[
+				'methods' => 'GET',
+				'callback' => [$this, 'get_push_settings'],
+				'permission_callback' => [$this, 'check_permission'],
+			],
+		]);
+		
+		// POST /woonoow/v1/notifications/push/settings
+		register_rest_route($this->namespace, '/' . $this->rest_base . '/push/settings', [
+			[
+				'methods' => 'POST',
+				'callback' => [$this, 'update_push_settings'],
+				'permission_callback' => [$this, 'check_permission'],
+			],
+		]);
 	}
 	
 	/**
@@ -468,4 +486,50 @@ class NotificationsController {
 			'message' => __('Unsubscribed from push notifications', 'woonoow'),
 		], 200);
 	}
+	
+	/**
+	 * Get push notification settings
+	 * 
+	 * @param WP_REST_Request $request Request object
+	 * @return WP_REST_Response
+	 */
+	public function get_push_settings(WP_REST_Request $request) {
+		$settings = PushNotificationHandler::get_settings();
+		
+		return new WP_REST_Response($settings, 200);
+	}
+	
+	/**
+	 * Update push notification settings
+	 * 
+	 * @param WP_REST_Request $request Request object
+	 * @return WP_REST_Response
+	 */
+	public function update_push_settings(WP_REST_Request $request) {
+		$settings = $request->get_json_params();
+		
+		if (empty($settings)) {
+			return new WP_Error(
+				'invalid_settings',
+				__('Settings data is required', 'woonoow'),
+				['status' => 400]
+			);
+		}
+		
+		$success = PushNotificationHandler::update_settings($settings);
+		
+		if (!$success) {
+			return new WP_Error(
+				'update_failed',
+				__('Failed to update push notification settings', 'woonoow'),
+				['status' => 500]
+			);
+		}
+		
+		return new WP_REST_Response([
+			'success' => true,
+			'message' => __('Push notification settings updated', 'woonoow'),
+			'settings' => PushNotificationHandler::get_settings(),
+		], 200);
+	}
 }