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/Shortcodes.php

@@ -27,13 +27,35 @@ class Shortcodes {
     private Plugin $plugin;
 
     /**
-     * Constructor.
+     * Whether running in unlicensed mode.
+     *
+     * @var bool
      */
-    public function __construct() {
-        $this->plugin = Plugin::get_instance();
+    private bool $unlicensed_mode = false;
+
+    /**
+     * Constructor.
+     *
+     * @param bool $unlicensed_mode Whether to run in unlicensed mode.
+     */
+    public function __construct( bool $unlicensed_mode = false ) {
+        $this->plugin          = Plugin::get_instance();
+        $this->unlicensed_mode = $unlicensed_mode;
         $this->register_shortcodes();
     }
 
+    /**
+     * Get the unlicensed message HTML.
+     *
+     * @return string
+     */
+    private function get_unlicensed_message(): string {
+        return '<div class="fedistream-unlicensed-notice" style="padding: 20px; background: #f0f0f1; border-left: 4px solid #dba617; margin: 10px 0;">'
+            . '<p style="margin: 0; color: #50575e;">'
+            . esc_html__( 'This content requires a valid FediStream license.', 'wp-fedistream' )
+            . '</p></div>';
+    }
+
     /**
      * Register all shortcodes.
      *
@@ -501,6 +523,11 @@ class Shortcodes {
      * @return string
      */
     private function render_template( string $template, array $context ): string {
+        // Check for unlicensed mode.
+        if ( $this->unlicensed_mode ) {
+            return $this->get_unlicensed_message();
+        }
+
         try {
             return $this->plugin->render( $template, $context );
         } catch ( \Exception $e ) {