feat(i18n): add full Arabic (ar) translation

Adds a complete Arabic locale (2759 keys, full parity with en) and wires
it into routing, RTL direction detection, message loading, the language
switcher, and flag icons alongside the existing he/fa RTL locales.

components/ui/language-switcher.tsx

@@ -8,6 +8,7 @@ import { flagComponents } from './flag-icons';
 
 const languages = [
   { value: 'auto', label: 'Auto' },
+  { value: 'ar', label: 'العربية' },
   { value: 'cs', label: 'Česky' },
   { value: 'sk', label: 'Slovenčina' },
   { value: 'da', label: 'Dansk' },