refactor(i18n): add locale to SUPPORTED_LANGUAGES and derive LOCALES from it

LOCALES is now built via Object.fromEntries from SUPPORTED_LANGUAGES,
so adding a new language only requires one change in supportedLanguages.ts.
Also types translations as Record<SupportedLanguageCode, ...> so TypeScript
enforces that every supported language has a translation entry.

client/src/i18n/supportedLanguages.ts

@@ -1,20 +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' },
+  { value: 'de',    label: 'Deutsch',             locale: 'de-DE' },
+  { value: 'en',    label: 'English',              locale: 'en-US' },
+  { value: 'es',    label: 'Español',              locale: 'es-ES' },
+  { value: 'fr',    label: 'Français',             locale: 'fr-FR' },
+  { value: 'hu',    label: 'Magyar',               locale: 'hu-HU' },
+  { value: 'nl',    label: 'Nederlands',           locale: 'nl-NL' },
+  { value: 'br',    label: 'Português (Brasil)',   locale: 'pt-BR' },
+  { value: 'cs',    label: 'Česky',                locale: 'cs-CZ' },
+  { value: 'pl',    label: 'Polski',               locale: 'pl-PL' },
+  { value: 'ru',    label: 'Русский',              locale: 'ru-RU' },
+  { value: 'zh',    label: '简体中文',              locale: 'zh-CN' },
+  { value: 'zh-TW', label: '繁體中文',              locale: 'zh-TW' },
+  { value: 'it',    label: 'Italiano',             locale: 'it-IT' },
+  { value: 'ar',    label: 'العربية',              locale: 'ar-SA' },
 ] as const
 
 export type SupportedLanguageCode = typeof SUPPORTED_LANGUAGES[number]['value']
 
-export const SUPPORTED_LANGUAGE_CODES = SUPPORTED_LANGUAGES.map(l => l.value)
+export const SUPPORTED_LANGUAGE_CODES: string[] = SUPPORTED_LANGUAGES.map(l => l.value)