feat: Upgrade to Next.js 16.1 & React 19.2, migrate to Supabase SSR with async client handling

src/app/loading.tsx

@@ -0,0 +1,12 @@
+import { Loader2 } from 'lucide-react';
+
+export default function Loading() {
+    return (
+        <div className="flex min-h-screen flex-col items-center justify-center p-6 bg-zinc-50 dark:bg-black text-center">
+            <div className="flex flex-col items-center gap-4">
+                <Loader2 size={40} className="animate-spin text-amber-600" />
+                <p className="text-zinc-500 font-medium animate-pulse">Whisky Vault wird geladen...</p>
+            </div>
+        </div>
+    );
+}