feat: add bilingual email notification system for key events

Notifications (fire-and-forget, non-blocking) for: review submitted,
approved/rejected/revision requested, task assigned/completed, issue
assigned/status changed, campaign created, user invited. Emails render
in user's preferred language (EN/AR) with RTL support. Adds
preferred_language to Users, syncs from frontend language toggle.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/src/i18n/LanguageContext.jsx

@@ -1,4 +1,5 @@
 import { createContext, useContext, useState, useEffect } from 'react'
+import api from '../utils/api'
 import en from './en.json'
 import ar from './ar.json'
 
@@ -33,6 +34,7 @@ export function LanguageProvider({ children }) {
     if (newLang !== 'en' && newLang !== 'ar') return
     setLangState(newLang)
     localStorage.setItem('digitalhub-lang', newLang)
+    api.patch('/api/users/me/language', { language: newLang }).catch(() => {})
   }
 
   const setCurrency = (code) => {