feat(login): add spinner animation during authentication

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/login.css

@@ -45,6 +45,10 @@
 .login-form__submit {
   width: 100%;
   margin-top: var(--space-2);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: var(--space-2);
 }
 
 .login-error {
@@ -63,3 +67,21 @@
   text-align: center;
   opacity: 0.6;
 }
+
+.login-spinner {
+  width: 16px;
+  height: 16px;
+  border: 2px solid color-mix(in srgb, currentColor 30%, transparent);
+  border-top-color: currentColor;
+  border-radius: 50%;
+  animation: login-spin 0.7s linear infinite;
+  flex-shrink: 0;
+}
+
+@keyframes login-spin {
+  to { transform: rotate(360deg); }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .login-spinner { animation: none; opacity: 0.6; }
+}