feat(09-02): wrap form-facing strings and localize JS validation

- Wrap all room names and 90+ furniture item names in __()
- Wrap all additional work section labels and field names in __()
- Wrap all form renderer UI strings (headers, labels, buttons) in gettext
- Pass translated address field labels to render methods
- Wrap validation error messages in class-form-handler
- Force German locale for email generation via switch_to_locale()
- Restore locale after email with restore_previous_locale()
- Add wp_localize_script() for JS validation messages
- Update form.js to use umzugslisteL10n for translated strings
- Email generator remains untouched (hardcoded German as required)
- Email subject stays hardcoded German (not wrapped in gettext)
- All PHP files pass syntax check
- 163 gettext calls in furniture-data, 26 in form-renderer

assets/js/form.js

@@ -9,6 +9,14 @@
 (function($) {
 	'use strict';
 
+	// Localized strings with fallbacks
+	var l10n = typeof umzugslisteL10n !== 'undefined' ? umzugslisteL10n : {
+		fieldRequired: 'This field is required',
+		invalidEmail: 'Please enter a valid email address',
+		selectMovingDate: 'Please select a complete moving date',
+		enterFurnitureItem: 'Please enter at least one furniture item'
+	};
+
 	/**
 	 * Parse German decimal format to float
 	 * Converts "0,40" or "0.40" to 0.40
@@ -224,14 +232,14 @@
 
 		// Check required fields
 		if (isRequired && !validateRequired(value)) {
-			showFieldError($field, 'Dieses Feld ist erforderlich');
+			showFieldError($field, l10n.fieldRequired);
 			return false;
 		}
 
 		// Check email format
 		if (fieldName === 'info[eE-Mail]' && value) {
 			if (!validateEmail(value)) {
-				showFieldError($field, 'Bitte geben Sie eine gültige E-Mail-Adresse ein');
+				showFieldError($field, l10n.invalidEmail);
 				return false;
 			}
 		}
@@ -282,7 +290,7 @@
 
 		// Validate date
 		if (!validateDate()) {
-			errors.push('Bitte wählen Sie ein vollständiges Umzugsdatum');
+			errors.push(l10n.selectMovingDate);
 			isValid = false;
 		}
 
@@ -295,7 +303,7 @@
 
 		// Validate furniture items
 		if (!validateFurnitureItems()) {
-			errors.push('Bitte geben Sie mindestens ein Möbelstück ein');
+			errors.push(l10n.enterFurnitureItem);
 			isValid = false;
 
 			// Scroll to first room table