feat(ui): add loading state for dashboard page

app/dashboard/loading.tsx

@@ -0,0 +1,20 @@
+export default function DashboardLoading() {
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-gradient-to-br from-slate-50 to-slate-100 dark:from-slate-900 dark:to-slate-800">
+      <div className="flex flex-col items-center space-y-4">
+        <div className="relative">
+          <div className="h-16 w-16 rounded-full border-4 border-slate-200 dark:border-slate-700"></div>
+          <div className="absolute inset-0 h-16 w-16 rounded-full border-4 border-transparent border-t-blue-500 animate-spin"></div>
+        </div>
+        <div className="text-center">
+          <h2 className="text-lg font-semibold text-slate-700 dark:text-slate-200">
+            Loading Dashboard
+          </h2>
+          <p className="text-sm text-slate-500 dark:text-slate-400">
+            Preparing your workspace...
+          </p>
+        </div>
+      </div>
+    </div>
+  );
+}