feat: add Farsi (fa) locale support

Add comprehensive Farsi translation for the webmail interface:
- Create locales/fa/common.json with Farsi translations
- Register 'fa' locale in i18n/routing.ts and i18n/request.ts
- Add Iran flag component (FlagIR) to flag-icons.tsx
- Add ف��رسی to language switcher dropdown
- Add Farsi language name to English locale for language selector

Translation covers login, sidebar, email viewer, composer, settings,
notifications, calendar, contacts, errors, shortcuts, and more.

i18n/request.ts

@@ -23,6 +23,9 @@ export default getRequestConfig(async ({ requestLocale }) => {
     case 'es':
       messages = (await import('../locales/es/common.json')).default;
       break;
+    case 'fa':
+      messages = (await import('../locales/fa/common.json')).default;
+      break;
     case 'fr':
       messages = (await import('../locales/fr/common.json')).default;
       break;