Add OIDC login flow to UI and fix dark mode datetime picker icons

- Add "Sign in with SSO" button on login page (shown when OIDC is configured)
- Add /oidc/callback route to exchange authorization code for JWT tokens
- Add loginWithOidcCode action to auth store
- Treat issuer URI as complete discovery URL (no auto-append of .well-known)
- Update admin page placeholder to show full discovery URL format
- Fix datetime picker calendar icon visibility in dark mode (color-scheme)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/auth/OidcCallback.tsx

@@ -0,0 +1,64 @@
+import { useEffect, useRef } from 'react';
+import { Navigate, useNavigate } from 'react-router';
+import { useAuthStore } from './auth-store';
+import styles from './LoginPage.module.css';
+
+export function OidcCallback() {
+  const { isAuthenticated, loading, error, loginWithOidcCode } = useAuthStore();
+  const navigate = useNavigate();
+  const exchanged = useRef(false);
+
+  useEffect(() => {
+    if (exchanged.current) return;
+    exchanged.current = true;
+
+    const params = new URLSearchParams(window.location.search);
+    const code = params.get('code');
+    const errorParam = params.get('error');
+
+    if (errorParam) {
+      useAuthStore.setState({
+        error: params.get('error_description') || errorParam,
+        loading: false,
+      });
+      return;
+    }
+
+    if (!code) {
+      useAuthStore.setState({ error: 'No authorization code received', loading: false });
+      return;
+    }
+
+    const redirectUri = `${window.location.origin}/oidc/callback`;
+    loginWithOidcCode(code, redirectUri);
+  }, [loginWithOidcCode]);
+
+  if (isAuthenticated) return <Navigate to="/" replace />;
+
+  return (
+    <div className={styles.page}>
+      <div className={styles.card}>
+        <div className={styles.logo}>
+          <svg width="22" height="22" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+            <path d="M12 2C6.5 2 2 6.5 2 12s4.5 10 10 10 10-4.5 10-10S17.5 2 12 2" />
+            <path d="M12 6v6l4 2" />
+          </svg>
+          cameleer3
+        </div>
+        {loading && <div className={styles.subtitle}>Completing sign-in...</div>}
+        {error && (
+          <>
+            <div className={styles.error}>{error}</div>
+            <button
+              className={styles.submit}
+              style={{ marginTop: 16 }}
+              onClick={() => navigate('/login')}
+            >
+              Back to Login
+            </button>
+          </>
+        )}
+      </div>
+    </div>
+  );
+}