Add theme toggle and button components to UI

Integrated a ThemeToggle component into the Navbar for dark/light mode switching and created a reusable Button component with configurable styles. Updated `.gitignore` to exclude the `lib` folder for cleanup.

frontend/src/components/layout/navbar.tsx

@@ -4,6 +4,7 @@
 import { useAuth } from "@/context/auth-context";
 import Link from "next/link";
 import { usePathname } from "next/navigation";
+import ThemeToggle from "@/components/ui/theme-toggle";
 
 export default function Navbar() {
   const { user, isAuthenticated, logout } = useAuth();
@@ -45,7 +46,9 @@ export default function Navbar() {
             )}
           </div>
 
-          <div className="flex items-center">
+          <div className="flex items-center space-x-4">
+            <ThemeToggle />
+
             {isAuthenticated ? (
               <div className="flex items-center space-x-4">
                 <span className="text-sm text-slate-600 dark:text-slate-300">