feat: Add SPA page selection UI in admin

Complete WooCommerce-style page architecture implementation:

Backend (already committed):
- API endpoint to fetch WordPress pages
- spa_pages field in appearance settings
- is_spa_page() checks in TemplateOverride and Assets

Frontend (this commit):
- Added page selector UI in Appearance > General
- Dropdowns for Shop, Cart, Checkout, Account pages
- Loads available WordPress pages from API
- Saves selected page IDs to settings
- Info alert explaining full-body rendering

UI Features:
- Clean page selection interface
- Shows all published WordPress pages
- '— None —' option to disable
- Integrated into existing General settings tab
- Follows existing design patterns

How it works:
1. Admin selects pages in Appearance > General
2. Page IDs saved to woonoow_appearance_settings
3. Frontend checks if current page matches selected pages
4. If match, renders full SPA to body (no theme interference)
5. Works with ANY theme consistently

Next: Test page selection and verify clean SPA rendering

includes/Admin/AppearanceController.php

@@ -56,6 +56,13 @@ class AppearanceController {
                 ],
             ],
         ]);
+        
+        // Get all WordPress pages for page selector
+        register_rest_route(self::API_NAMESPACE, '/pages/list', [
+            'methods' => 'GET',
+            'callback' => [__CLASS__, 'get_pages_list'],
+            'permission_callback' => [__CLASS__, 'check_permission'],
+        ]);
     }
     
     public static function check_permission() {
@@ -377,6 +384,30 @@ class AppearanceController {
         return $sanitized;
     }
     
+    /**
+     * Get list of WordPress pages for page selector
+     */
+    public static function get_pages_list(WP_REST_Request $request) {
+        $pages = get_pages([
+            'post_status' => 'publish',
+            'sort_column' => 'post_title',
+            'sort_order' => 'ASC',
+        ]);
+        
+        $pages_list = array_map(function($page) {
+            return [
+                'id' => $page->ID,
+                'title' => $page->post_title,
+                'slug' => $page->post_name,
+            ];
+        }, $pages);
+        
+        return new WP_REST_Response([
+            'success' => true,
+            'data' => $pages_list,
+        ], 200);
+    }
+    
     /**
      * Get default settings structure
      */