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/router.tsx

@@ -2,6 +2,7 @@ import { createBrowserRouter, Navigate } from 'react-router';
 import { AppShell } from './components/layout/AppShell';
 import { ProtectedRoute } from './auth/ProtectedRoute';
 import { LoginPage } from './auth/LoginPage';
+import { OidcCallback } from './auth/OidcCallback';
 import { ExecutionExplorer } from './pages/executions/ExecutionExplorer';
 import { OidcAdminPage } from './pages/admin/OidcAdminPage';
 
@@ -10,6 +11,10 @@ export const router = createBrowserRouter([
     path: '/login',
     element: <LoginPage />,
   },
+  {
+    path: '/oidc/callback',
+    element: <OidcCallback />,
+  },
   {
     element: <ProtectedRoute />,
     children: [