feat: add test email admin page and comprehensive form fill button

- New Umzugsliste_Test_Email class with test data generator covering all
  fields: addresses, 7 rooms with items, all additional work sections, sonstiges
- Admin page under Moving List > Test Email with inline preview iframe
  and Send Test Email button (manage_options capability)
- Replace Step 1 dev fill button with Fill All that populates every field
  across all 9 steps (furniture, additional work, sonstiges)
- Fix getFieldVal crash when select has no selection (selectedIndex=-1)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

umzugsliste.php

@@ -84,6 +84,7 @@ class Umzugsliste {
 		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-form-renderer.php';
 		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-shortcode.php';
 		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-email-generator.php';
+		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-test-email.php';
 		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-form-handler.php';
 	}
 
@@ -152,9 +153,10 @@ class Umzugsliste {
 		$cpt = Umzugsliste_CPT::get_instance();
 		$cpt->register_post_type();
 
-		// Initialize admin menu
+		// Initialize admin menu and test email
 		if ( is_admin() ) {
 			Umzugsliste_Admin_Menu::get_instance();
+			Umzugsliste_Test_Email::get_instance();
 		}
 
 		// Initialize settings