Add login functionality, authentication handling, and logout support

Replaced React 19 with React 18 due to compatibility issues. Introduced conditional rendering for login and app components based on authentication state. Added basic login form and error handling. Updated styles for authentication UI and enhanced header with logout functionality.

src/styles/components.css

@@ -671,4 +671,48 @@ textarea {
   overflow-y: auto;
   border: 1px solid #e2e8f0;
   font-size: 0.9rem;
-}
+}
+
+/* Auth / Login Styles */
+.auth-wrapper {
+  min-height: 100vh;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 40px 20px;
+}
+
+.auth-card {
+  width: 100%;
+  max-width: 420px;
+}
+
+.btn-block {
+  width: 100%;
+  display: inline-flex;
+}
+
+.alert {
+  padding: 12px 14px;
+  border-radius: 8px;
+  font-size: 0.95rem;
+  margin: 10px 0 16px;
+  border-left: 4px solid transparent;
+}
+
+.alert-error {
+  background: rgba(239, 68, 68, 0.08);
+  color: #991b1b;
+  border-left-color: var(--danger);
+}
+
+/* Improve form spacing inside auth card */
+.auth-card .form-group {
+  margin-bottom: 18px;
+}
+
+@media (max-width: 480px) {
+  .auth-card {
+    max-width: 100%;
+  }
+}