feat(web): add same-origin SPA authentication

apps/web/src/routes.tsx

@@ -1,14 +1,33 @@
-import { createBrowserRouter, Navigate, type RouteObject } from 'react-router-dom';
+import type { ReactElement } from 'react';
+import { createBrowserRouter, Navigate, Outlet, type RouteObject } from 'react-router-dom';
+import { LoginPage } from '@/spa/pages/login';
+import { RegisterPage } from '@/spa/pages/register';
+import { SsoCallbackPage } from '@/spa/pages/sso-callback';
 import { AuthGuard, GuestGuard } from '@/spa/guards';
 import { Placeholder } from '@/spa/placeholder';
 
+function GuestLayout(): ReactElement {
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-surface-bg px-4 py-8">
+      <div className="w-full max-w-md rounded-xl border border-surface-border bg-surface-card p-8 shadow-lg">
+        <Outlet />
+      </div>
+    </div>
+  );
+}
+
 export const routes: RouteObject[] = [
   {
     element: <GuestGuard />,
     children: [
-      { path: '/login', element: <Placeholder title="Login" /> },
-      { path: '/register', element: <Placeholder title="Register" /> },
-      { path: '/auth/provider/:provider', element: <Placeholder title="Signing in" /> },
+      {
+        element: <GuestLayout />,
+        children: [
+          { path: '/login', element: <LoginPage /> },
+          { path: '/register', element: <RegisterPage /> },
+          { path: '/auth/provider/:provider', element: <SsoCallbackPage /> },
+        ],
+      },
     ],
   },
   {