refactor(i18n): extract SUPPORTED_LANGUAGES to avoid duplication

Move language list to supportedLanguages.ts so TranslationContext and
settingsStore can import from a single source of truth, eliminating
the hardcoded array in setLanguageTransient.

client/src/i18n/supportedLanguages.ts

@@ -0,0 +1,20 @@
+export const SUPPORTED_LANGUAGES = [
+  { value: 'de', label: 'Deutsch' },
+  { value: 'en', label: 'English' },
+  { value: 'es', label: 'Español' },
+  { value: 'fr', label: 'Français' },
+  { value: 'hu', label: 'Magyar' },
+  { value: 'nl', label: 'Nederlands' },
+  { value: 'br', label: 'Português (Brasil)' },
+  { value: 'cs', label: 'Česky' },
+  { value: 'pl', label: 'Polski' },
+  { value: 'ru', label: 'Русский' },
+  { value: 'zh', label: '简体中文' },
+  { value: 'zh-TW', label: '繁體中文' },
+  { value: 'it', label: 'Italiano' },
+  { value: 'ar', label: 'العربية' },
+] as const
+
+export type SupportedLanguageCode = typeof SUPPORTED_LANGUAGES[number]['value']
+
+export const SUPPORTED_LANGUAGE_CODES = SUPPORTED_LANGUAGES.map(l => l.value)