feat(localization): expand language support and enhance UI for user settings

- Added support for additional UI locales including Cebuano and Spanish, improving accessibility for a broader user base.
- Updated language selection components in the AppHeader and SettingsWidget to reflect new language options, enhancing user experience.
- Enhanced localization of various UI elements across components, ensuring consistent language representation and improved user engagement.
- Implemented logic to synchronize user language preferences with backend settings, providing a seamless experience when changing languages.

frontend/src/store/index.js

@@ -30,6 +30,8 @@ function clearAuthStorage() {
   });
 }
 
+const SUPPORTED_UI_LOCALES = ['de', 'en', 'ceb', 'es'];
+
 function persistAuthStorage(user, rememberMe) {
   const targetStorage = rememberMe ? localStorage : sessionStorage;
   clearAuthStorage();
@@ -38,6 +40,15 @@ function persistAuthStorage(user, rememberMe) {
   targetStorage.setItem('userid', user?.id || '');
 }
 
+function readPersistedUiLanguage() {
+  try {
+    const saved = localStorage.getItem('uiLanguage');
+    return saved && SUPPORTED_UI_LOCALES.includes(saved) ? saved : null;
+  } catch (_) {
+    return null;
+  }
+}
+
 const store = createStore({
   state: {
     isLoggedIn: getStoredValue('isLoggedIn') === 'true',
@@ -52,7 +63,10 @@ const store = createStore({
     backendConnecting: false,
     daemonConnecting: false,
     language: (() => {
-      // Verwende die gleiche Logik wie in main.js
+      const persisted = readPersistedUiLanguage();
+      if (persisted) {
+        return persisted;
+      }
       const browserLanguage = navigator.language || navigator.languages[0];
 
       if (browserLanguage.startsWith('ceb') || browserLanguage.startsWith('bis')) {
@@ -99,6 +113,15 @@ const store = createStore({
       state.user = user;
       persistAuthStorage(user, rememberMe);
       state.menuNeedsUpdate = true;
+      const langParam = user.param?.find((p) => p.name === 'language');
+      if (langParam?.value && SUPPORTED_UI_LOCALES.includes(langParam.value)) {
+        state.language = langParam.value;
+        try {
+          localStorage.setItem('uiLanguage', langParam.value);
+        } catch (_) {
+          /* ignore */
+        }
+      }
       if (user.param.filter(param => ['birthdate', 'gender'].includes(param.name)).length < 2) {
         router.push({ path: '/settings/personal' });
       }
@@ -110,6 +133,11 @@ const store = createStore({
       localStorage.removeItem('menu');
       state.menuNeedsUpdate = false;
       
+      try {
+        localStorage.removeItem('uiLanguage');
+      } catch (_) {
+        /* ignore */
+      }
       // Setze die Sprache auf die Browser-Sprache zurück
       const browserLanguage = navigator.language || navigator.languages[0];
 
@@ -150,6 +178,11 @@ const store = createStore({
     },
     setLanguage(state, language) {
       state.language = language;
+      try {
+        localStorage.setItem('uiLanguage', language);
+      } catch (_) {
+        /* ignore */
+      }
     },
     setMenu(state, menu) {
       state.menu = menu;