feat: Add license management and tabbed settings (v0.3.0)

- Implement license management using magdev/wc-licensed-product-client
- Reorganize settings page into License, Default Settings, Integrations tabs
- Add license validation and activation via AJAX
- Frontend features require valid license (admin works always)
- Update translations with German (de_CH) for license strings

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

includes/Frontend/Ajax.php

@@ -7,6 +7,8 @@
 
 namespace WP_FediStream\Frontend;
 
+use WP_FediStream\License\Manager as LicenseManager;
+
 // Prevent direct file access.
 if ( ! defined( 'ABSPATH' ) ) {
     exit;
@@ -36,6 +38,11 @@ class Ajax {
      * @return void
      */
     public function get_track(): void {
+        // Check license.
+        if ( ! LicenseManager::is_license_valid() ) {
+            wp_send_json_error( array( 'message' => __( 'This feature requires a valid license.', 'wp-fedistream' ) ) );
+        }
+
         // Verify nonce.
         if ( ! isset( $_POST['nonce'] ) || ! wp_verify_nonce( sanitize_key( $_POST['nonce'] ), 'wp-fedistream-nonce' ) ) {
             wp_send_json_error( array( 'message' => __( 'Invalid nonce.', 'wp-fedistream' ) ) );
@@ -125,6 +132,11 @@ class Ajax {
      * @return void
      */
     public function record_play(): void {
+        // Check license.
+        if ( ! LicenseManager::is_license_valid() ) {
+            wp_send_json_error( array( 'message' => __( 'This feature requires a valid license.', 'wp-fedistream' ) ) );
+        }
+
         // Verify nonce.
         if ( ! isset( $_POST['nonce'] ) || ! wp_verify_nonce( sanitize_key( $_POST['nonce'] ), 'wp-fedistream-nonce' ) ) {
             wp_send_json_error( array( 'message' => __( 'Invalid nonce.', 'wp-fedistream' ) ) );