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.

i18n/direction.ts

@@ -1,5 +1,5 @@
-// RTL locales: Hebrew (he) and Persian/Farsi (fa). Everything else is LTR.
-const rtlLocales = new Set(['he', 'fa']);
+// RTL locales: Arabic (ar), Hebrew (he), and Persian/Farsi (fa). Everything else is LTR.
+const rtlLocales = new Set(['ar', 'he', 'fa']);
 
 export function getLocaleDirection(locale: string): 'ltr' | 'rtl' {
   return rtlLocales.has(locale) ? 'rtl' : 'ltr';