Add loading.js and error.js for all routes, secure admin page with 404

- Added loading.js (dark/light variants) for all 14 routes
- Added error.js (dark/light variants) for all 14 routes
- Added global not-found.js and loading.js at root
- Admin page shows 404 illustration for non-admin users instead of redirecting

app/loading.js

@@ -0,0 +1,18 @@
+'use client';
+
+import { motion } from 'framer-motion';
+
+export default function Loading() {
+  return (
+    <div className="min-h-screen bg-gray-50 flex items-center justify-center">
+      <motion.div
+        initial={{ opacity: 0 }}
+        animate={{ opacity: 1 }}
+        className="text-center"
+      >
+        <div className="w-16 h-16 border-4 border-amber-500 border-t-transparent rounded-full animate-spin mx-auto mb-4" />
+        <p className="text-gray-500 text-lg">جاري التحميل...</p>
+      </motion.div>
+    </div>
+  );
+}