Add Authentik OIDC PKCE login and refusal handling to admin PWA

admin/src/App.css

@@ -67,4 +67,51 @@
 .admin-tab.active {
   font-weight: 700;
   color: var(--nq-brand-blue);
-}
+}
+/* Auth screens (login, errors, refusal). */
+
+.admin-login,
+.admin-error,
+.admin-refusal,
+.admin-loading {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 10px;
+  min-height: 100%;
+  padding: 32px 16px;
+  text-align: center;
+}
+
+.admin-login-button {
+  margin-top: 8px;
+  padding: 12px 24px;
+  border: none;
+  border-radius: 8px;
+  background: var(--nq-a-accent, #2563eb);
+  color: #fff;
+  font-size: 15px;
+  font-weight: 700;
+  cursor: pointer;
+}
+
+.admin-error,
+.admin-refusal {
+  color: var(--nq-a-ink, inherit);
+}
+
+.admin-refusal h1,
+.admin-error h1 {
+  margin: 0;
+  font-size: 20px;
+}
+
+.admin-refusal p,
+.admin-error p {
+  margin: 0;
+  max-width: 34em;
+  font-size: 14px;
+  font-weight: 600;
+  color: var(--nq-a-ink-secondary);
+}