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.

components/ui/language-switcher.tsx

@@ -12,6 +12,7 @@ const languages = [
   { value: 'da', label: 'Dansk' },
   { value: 'de', label: 'Deutsch' },
   { value: 'en', label: 'English' },
+  { value: 'fa', label: 'فارسی' },
   { value: 'es', label: 'Español' },
   { value: 'fr', label: 'Français' },
   { value: 'it', label: 'Italiano' },