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.
