Handle restricted language configuration fallback V2 (#5154)

## Summary
- restrict supported languages to the validated list from app-config
instead of always adding an extra fallback
- set the effective fallback locale to the preferred configured language
and switch away from disallowed selections automatically

## Testing
- ./gradlew build


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_b_6930529bc6c08328a1ce05f7d1316e27)

frontend/src/proprietary/components/shared/config/configSections/AdminGeneralSection.tsx

@@ -1,6 +1,6 @@
-import { useEffect, useState, useRef, useCallback } from 'react';
+import { useEffect, useState, useRef, useCallback, useMemo } from 'react';
 import { useTranslation } from 'react-i18next';
-import { TextInput, Switch, Button, Stack, Paper, Text, Loader, Group, MultiSelect, Badge, SegmentedControl } from '@mantine/core';
+import { TextInput, Switch, Button, Stack, Paper, Text, Loader, Group, MultiSelect, Badge, SegmentedControl, Select } from '@mantine/core';
 import { alert } from '@app/components/toast';
 import RestartConfirmationModal from '@app/components/shared/config/RestartConfirmationModal';
 import { useRestartServer } from '@app/components/shared/config/useRestartServer';
@@ -11,6 +11,8 @@ import { useLoginRequired } from '@app/hooks/useLoginRequired';
 import LoginRequiredBanner from '@app/components/shared/config/LoginRequiredBanner';
 import { usePreferences } from '@app/contexts/PreferencesContext';
 import { useUnsavedChanges } from '@app/contexts/UnsavedChangesContext';
+import { supportedLanguages, toUnderscoreFormat, toUnderscoreLanguages } from '@app/i18n';
+import { Z_INDEX_CONFIG_MODAL } from '@app/styles/zIndex';
 
 interface GeneralSettingsData {
   ui: {
@@ -49,6 +51,12 @@ export default function AdminGeneralSection() {
   const { restartModalOpened, showRestartModal, closeRestartModal, restartServer } = useRestartServer();
   const { preferences, updatePreference } = usePreferences();
   const { setIsDirty, markClean } = useUnsavedChanges();
+  const languageOptions = useMemo(
+    () => Object.entries(supportedLanguages)
+      .map(([code, label]) => ({ value: toUnderscoreFormat(code), label: `${label} (${code})` }))
+      .sort((a, b) => a.label.localeCompare(b.label)),
+    []
+  );
   
   // Track original settings for dirty detection
   const [originalSettingsSnapshot, setOriginalSettingsSnapshot] = useState<string>('');
@@ -73,9 +81,11 @@ export default function AdminGeneralSection() {
         apiClient.get('/api/v1/admin/settings/section/premium')
       ]);
 
-      const ui = uiResponse.data || {};
-      const system = systemResponse.data || {};
-      const premium = premiumResponse.data || {};
+      const ui = { ...(uiResponse.data || {}) };
+      const system = { ...(systemResponse.data || {}) };
+      const premium = { ...(premiumResponse.data || {}) };
+
+      ui.languages = Array.isArray(ui.languages) ? toUnderscoreLanguages(ui.languages) : [];
 
       const result: any = {
         ui,
@@ -152,6 +162,21 @@ export default function AdminGeneralSection() {
     }
   });
 
+  const selectedLanguages = useMemo(
+    () => toUnderscoreLanguages(settings.ui?.languages || []),
+    [settings.ui?.languages]
+  );
+
+  // Filter default locale options based on available languages setting
+  const defaultLocaleOptions = useMemo(() => {
+    // If no languages are selected (empty), show all languages
+    if (!selectedLanguages || selectedLanguages.length === 0) {
+      return languageOptions;
+    }
+    // Otherwise, only show languages that are in the selected list
+    return languageOptions.filter(option => selectedLanguages.includes(option.value));
+  }, [selectedLanguages, languageOptions]);
+
   useEffect(() => {
     // Only fetch real settings if login is enabled
     if (loginEnabled) {
@@ -369,30 +394,19 @@ export default function AdminGeneralSection() {
                 </Group>
               }
               description={t('admin.settings.general.languages.description', 'Limit which languages are available (empty = all languages)')}
-              value={settings.ui?.languages || []}
+              value={selectedLanguages}
               onChange={(value) => setSettings({ ...settings, ui: { ...settings.ui, languages: value } })}
-              data={[
-                { value: 'de_DE', label: 'Deutsch' },
-                { value: 'es_ES', label: 'Español' },
-                { value: 'fr_FR', label: 'Français' },
-                { value: 'it_IT', label: 'Italiano' },
-                { value: 'pl_PL', label: 'Polski' },
-                { value: 'pt_BR', label: 'Português (Brasil)' },
-                { value: 'ru_RU', label: 'Русский' },
-                { value: 'zh_CN', label: '简体中文' },
-                { value: 'ja_JP', label: '日本語' },
-                { value: 'ko_KR', label: '한국어' },
-              ]}
+              data={languageOptions}
               searchable
               clearable
               placeholder={t('admin.settings.general.languages.placeholder', 'Select languages')}
-              comboboxProps={{ zIndex: 1400 }}
+              comboboxProps={{ zIndex: Z_INDEX_CONFIG_MODAL }}
               disabled={!loginEnabled}
             />
           </div>
 
           <div>
-            <TextInput
+            <Select
               label={
                 <Group gap="xs">
                   <span>{t('admin.settings.general.defaultLocale.label', 'Default Locale')}</span>
@@ -400,9 +414,13 @@ export default function AdminGeneralSection() {
                 </Group>
               }
               description={t('admin.settings.general.defaultLocale.description', 'The default language for new users (e.g., en_US, es_ES)')}
-              value={ settings.system?.defaultLocale || ''}
-              onChange={(e) => setSettings({ ...settings, system: { ...settings.system, defaultLocale: e.target.value } })}
-              placeholder="en_US"
+              value={settings.system?.defaultLocale || ''}
+              onChange={(value) => setSettings({ ...settings, system: { ...settings.system, defaultLocale: value || '' } })}
+              data={defaultLocaleOptions}
+              searchable
+              clearable
+              placeholder="en_GB"
+              comboboxProps={{ zIndex: Z_INDEX_CONFIG_MODAL }}
               disabled={!loginEnabled}
             />
           </div>