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>

assets/js/form.js

@@ -409,7 +409,7 @@
 	function getFieldVal(name) {
 		var el = qs('[name="' + name + '"]');
 		if (!el) return '';
-		if (el.tagName === 'SELECT') return el.options[el.selectedIndex].value;
+		if (el.tagName === 'SELECT') return el.selectedIndex >= 0 ? el.options[el.selectedIndex].value : '';
 		return el.value.trim();
 	}