feat: Integrate Kit OAuth v4 API and refactor admin architecture

- Add Kit OAuth v4 authentication with shared ConvertKit libraries
- Replace API key/secret with OAuth flow using official Kit WordPress client
- Refactor Core class to Main with enhanced error handling and boot error notices
- Implement Admin_Notices_API and Admin_Banner for improved admin UI
- Add connection testing for Kit API credentials via AJAX
- Introduce Freemius event type configuration for free/paid user mappings

Author: ajaydsouza

glue-link.php

@@ -41,18 +41,43 @@
 	define( 'GLUE_LINK_PLUGIN_URL', plugin_dir_url( __FILE__ ) );
 }
 
+// Kit OAuth defaults (same OAuth application used by the official Kit WordPress flow).
+if ( ! defined( 'GLUE_LINK_KIT_OAUTH_CLIENT_ID' ) ) {
+	define( 'GLUE_LINK_KIT_OAUTH_CLIENT_ID', 'HXZlOCj-K5r0ufuWCtyoyo3f688VmMAYSsKg1eGvw0Y' );
+}
+if ( ! defined( 'GLUE_LINK_KIT_OAUTH_REDIRECT_URI' ) ) {
+	define( 'GLUE_LINK_KIT_OAUTH_REDIRECT_URI', 'https://app.kit.com/wordpress/redirect' );
+}
+
+// Load Kit shared library classes if not already loaded by another plugin.
+if ( ! trait_exists( 'ConvertKit_API_Traits' ) ) {
+	require_once GLUE_LINK_PLUGIN_DIR . 'vendor/convertkit/convertkit-wordpress-libraries/src/class-convertkit-api-traits.php';
+}
+if ( ! class_exists( 'ConvertKit_API_V4' ) ) {
+	require_once GLUE_LINK_PLUGIN_DIR . 'vendor/convertkit/convertkit-wordpress-libraries/src/class-convertkit-api-v4.php';
+}
+if ( ! class_exists( 'ConvertKit_Log' ) ) {
+	require_once GLUE_LINK_PLUGIN_DIR . 'vendor/convertkit/convertkit-wordpress-libraries/src/class-convertkit-log.php';
+}
+if ( ! class_exists( 'ConvertKit_Resource_V4' ) ) {
+	require_once GLUE_LINK_PLUGIN_DIR . 'vendor/convertkit/convertkit-wordpress-libraries/src/class-convertkit-resource-v4.php';
+}
+if ( ! class_exists( 'ConvertKit_Review_Request' ) ) {
+	require_once GLUE_LINK_PLUGIN_DIR . 'vendor/convertkit/convertkit-wordpress-libraries/src/class-convertkit-review-request.php';
+}
+
 // Autoloader.
 require_once GLUE_LINK_PLUGIN_DIR . 'includes/autoloader.php';
 
 // Register activation hook.
-register_activation_hook( __FILE__, array( 'WebberZone\Glue_Link\Core', 'activate' ) );
+register_activation_hook( __FILE__, array( 'WebberZone\Glue_Link\Main', 'activate' ) );
 
 /**
  * Global variable holding the current instance of Glue Link
  *
  * @since 1.0.0
  *
- * @var \WebberZone\Glue_Link\Core
+ * @var \WebberZone\Glue_Link\Main
  */
 global $glue_link;
 
@@ -65,18 +90,61 @@
 	 * @return void
 	 */
 	function load() {
+		glue_link();
+	}
+	add_action( 'plugins_loaded', __NAMESPACE__ . '\\load' );
+}
+
+if ( ! function_exists( __NAMESPACE__ . '\\glue_link' ) ) {
+	/**
+	 * Get the main Glue Link instance.
+	 *
+	 * @since 1.0.0
+	 * @return Main Main instance.
+	 */
+	function glue_link() {
 		global $glue_link;
-		$glue_link = Core::get_instance();
+		delete_option( 'glue_link_boot_error' );
+
+		try {
+			$glue_link = Main::get_instance();
+		} catch ( \Throwable $e ) {
+			update_option(
+				'glue_link_boot_error',
+				sprintf(
+					/* translators: 1: Error message, 2: File, 3: Line. */
+					__( 'Glue Link failed to initialize: %1$s in %2$s on line %3$d', 'glue-link' ),
+					$e->getMessage(),
+					$e->getFile(),
+					$e->getLine()
+				)
+			);
+		}
+
+		return $glue_link;
 	}
 }
-add_action( 'plugins_loaded', __NAMESPACE__ . '\\load' );
 
 /**
- * Global variable holding the current settings for Glue Link
+ * Show plugin bootstrap errors in wp-admin.
  *
  * @since 1.0.0
  *
- * @var array<string, mixed>
+ * @return void
  */
-global $glue_link_settings;
-$glue_link_settings = Options_API::get_settings();
+function show_boot_error_notice() {
+	if ( ! current_user_can( 'manage_options' ) ) {
+		return;
+	}
+
+	$error = get_option( 'glue_link_boot_error' );
+	if ( empty( $error ) ) {
+		return;
+	}
+
+	printf(
+		'<div class="notice notice-error"><p>%s</p></div>',
+		esc_html( (string) $error )
+	);
+}
+add_action( 'admin_notices', __NAMESPACE__ . '\\show_boot_error_notice' );