improvement: sidebar refactored. COMMING SOON page added for all non-developed tabs.

frontend/src/app/(dashboard)/staff/page.tsx

@@ -0,0 +1,10 @@
+export default function StaffPage() {
+  return (
+    <div className="space-y-3">
+      <h1 className="text-2xl font-semibold text-text-primary">Staff Management</h1>
+      <p className="text-sm text-text-secondary">
+        Staff management module is coming soon.
+      </p>
+    </div>
+  );
+}