fix(ux): locale reload feedback, submit validation, dedup blur logic

- Locale-Picker: disable select + fade before location.reload() (system mode)
  gives visual feedback before the page jumps
- Extract _validateField() helper from wireBlurValidation to avoid duplication
- Add validateAll(formContainer): validates all required fields on demand,
  marks inline errors, focuses first invalid field
- tasks.js: call validateAll() at submit to catch untouched required fields

public/pages/tasks.js

@@ -6,7 +6,7 @@
 
 import { api } from '/api.js';
 import { renderRRuleFields, bindRRuleEvents, getRRuleValues } from '/rrule-ui.js';
-import { openModal as openSharedModal, closeModal, wireBlurValidation, btnSuccess, btnError, promptModal, confirmModal } from '/components/modal.js';
+import { openModal as openSharedModal, closeModal, wireBlurValidation, validateAll, btnSuccess, btnError, promptModal, confirmModal } from '/components/modal.js';
 import { stagger, vibrate } from '/utils/ux.js';
 import { t, formatDate } from '/i18n.js';
 import { esc } from '/utils/html.js';
@@ -430,6 +430,9 @@ async function handleFormSubmit(e, container) {
   const submitBtn = document.getElementById('task-submit-btn');
   const taskId    = document.getElementById('task-id').value;
 
+  // Alle required-Felder sofort validieren (auch unberührte)
+  if (!validateAll(form)) return;
+
   errorEl.hidden = true;
   submitBtn.disabled = true;
   submitBtn.textContent = t('common.saving');