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/components/oikos-locale-picker.js

@@ -55,8 +55,11 @@ class OikosLocalePicker extends HTMLElement {
 
     select.addEventListener('change', () => {
       if (select.value === 'system') {
+        select.disabled = true;
+        select.style.opacity = '0.5';
         localStorage.removeItem('oikos-locale');
-        location.reload();
+        // Kurze Verzögerung damit der Browser den disabled-Zustand rendert
+        setTimeout(() => location.reload(), 60);
       } else {
         setLocale(select.value);
       }