fix: replace hardcoded localhost:3001 URLs with relative paths

Fixes CORS errors when deployed behind a reverse proxy by removing
hardcoded localhost references in Brands.jsx and PostCard.jsx.

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

client/src/pages/Brands.jsx

@@ -7,7 +7,7 @@ import { AppContext } from '../App'
 import Modal from '../components/Modal'
 import { SkeletonCard } from '../components/SkeletonLoader'
 
-const API_BASE = import.meta.env.VITE_API_URL || 'http://localhost:3001/api'
+const API_BASE = '/api'
 
 const EMPTY_BRAND = { name: '', name_ar: '', priority: 2, icon: '' }