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.

locales/en/common.json

@@ -804,7 +804,10 @@
     "pt": "Português",
     "ru": "Русский",
     "uk": "Українська",
-    "zh": "简体中文"
+    "zh": "简体中文",
+    "fa": "فارسی",
+    "farsi": "فارسی",
+    "switch_to_farsi": "Switch to Farsi"
   },
   "settings": {
     "title": "Settings",