fix: resolve form submission issues and add CPT detail view

- Rename day/month/year form fields to umzug_day/umzug_month/umzug_year
  to avoid conflict with WordPress reserved public query variables that
  caused a 404 on form POST
- Move form handler instantiation to init_hooks() so handle_submission
  registers on the init action before it fires
- Add standalone template fallback: detect [umzugsliste] shortcode in
  page content when form_page_id option is not set
- Add submission details meta box to CPT entries showing addresses,
  furniture, additional work, and status
- Add German translations for all new admin meta box strings

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

umzugsliste.php

@@ -93,6 +93,9 @@ class Umzugsliste {
 	private function init_hooks() {
 		add_action( 'init', array( $this, 'init' ) );
 		add_filter( 'template_include', array( $this, 'maybe_load_form_template' ) );
+
+		// Form handler must register its init hook before init fires
+		Umzugsliste_Form_Handler::get_instance();
 	}
 
 	/**
@@ -106,9 +109,23 @@ class Umzugsliste {
 			return $template;
 		}
 
-		$form_page_id = (int) get_option( 'umzugsliste_form_page_id', 0 );
+		$use_standalone = false;
 
+		// Check configured form page ID
+		$form_page_id = (int) get_option( 'umzugsliste_form_page_id', 0 );
 		if ( $form_page_id > 0 && is_page( $form_page_id ) ) {
+			$use_standalone = true;
+		}
+
+		// Fallback: check if current page contains the [umzugsliste] shortcode
+		if ( ! $use_standalone ) {
+			$post = get_queried_object();
+			if ( $post && isset( $post->post_content ) && has_shortcode( $post->post_content, 'umzugsliste' ) ) {
+				$use_standalone = true;
+			}
+		}
+
+		if ( $use_standalone ) {
 			$custom_template = UMZUGSLISTE_PLUGIN_DIR . 'templates/form-page.php';
 			if ( file_exists( $custom_template ) ) {
 				return $custom_template;
@@ -136,9 +153,6 @@ class Umzugsliste {
 
 		// Initialize shortcode
 		Umzugsliste_Shortcode::get_instance();
-
-		// Initialize form handler
-		Umzugsliste_Form_Handler::get_instance();
 	}
 }