feat(web): add same-origin SPA authentication

apps/web/src/app/auth/provider/[provider]/page.tsx

@@ -3,41 +3,56 @@
 import Link from 'next/link';
 import { useEffect, useState } from 'react';
 import { useParams, useSearchParams } from 'next/navigation';
+import { api } from '@/lib/api';
+import { resolveAuthCallbackURL } from '@/lib/auth-redirect';
 import { signIn } from '@/lib/auth-client';
-import { getSsoProvider } from '@/lib/sso-providers';
+import type { SsoProviderDiscovery } from '@/lib/sso';
 
 export default function AuthProviderRedirectPage(): React.ReactElement {
   const params = useParams<{ provider: string }>();
   const searchParams = useSearchParams();
   const providerId = typeof params.provider === 'string' ? params.provider : '';
-  const provider = getSsoProvider(providerId);
-  const callbackURL = searchParams.get('callbackURL') ?? '/chat';
+  const requestedCallbackURL = searchParams.get('callbackURL');
+  const [providerName, setProviderName] = useState<string | null>(null);
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
-    const currentProvider = provider;
-
-    if (!currentProvider) {
-      setError('Unknown SSO provider.');
-      return;
-    }
-
-    if (!currentProvider.enabled) {
-      setError(`${currentProvider.buttonLabel} is not enabled in this deployment.`);
-      return;
-    }
-
-    const activeProvider = currentProvider;
     let cancelled = false;
 
     async function redirectToProvider(): Promise<void> {
-      const result = await signIn.oauth2({
-        providerId: activeProvider.id,
-        callbackURL,
-      });
+      try {
+        const callbackURL = resolveAuthCallbackURL(requestedCallbackURL, window.location.origin);
+        const providers = await api<SsoProviderDiscovery[]>('/api/sso/providers');
+        if (cancelled) return;
 
-      if (!cancelled && result?.error) {
-        setError(result.error.message ?? `${activeProvider.buttonLabel} sign in failed.`);
+        const provider = providers.find((candidate) => candidate.id === providerId);
+        if (!provider) {
+          setError('Unknown SSO provider.');
+          return;
+        }
+
+        setProviderName(provider.name);
+        if (!provider.configured) {
+          setError(`${provider.name} is not enabled in this deployment.`);
+          return;
+        }
+        if (provider.loginMode !== 'oidc') {
+          setError(`${provider.name} is not available for OIDC sign in.`);
+          return;
+        }
+
+        const result = await signIn.oauth2({
+          providerId: provider.id,
+          callbackURL,
+        });
+
+        if (!cancelled && result?.error) {
+          setError(result.error.message ?? `${provider.name} sign in failed.`);
+        }
+      } catch (caught: unknown) {
+        if (!cancelled) {
+          setError(caught instanceof Error ? caught.message : 'Unable to start single sign-on.');
+        }
       }
     }
 
@@ -46,19 +61,22 @@ export default function AuthProviderRedirectPage(): React.ReactElement {
     return () => {
       cancelled = true;
     };
-  }, [callbackURL, provider]);
+  }, [providerId, requestedCallbackURL]);
 
   return (
     <div className="mx-auto flex min-h-[50vh] max-w-md flex-col justify-center">
       <h1 className="text-2xl font-semibold text-text-primary">Single sign-on</h1>
       <p className="mt-2 text-sm text-text-secondary">
-        {provider
-          ? `Redirecting you to ${provider.buttonLabel.replace('Continue with ', '')}...`
+        {providerName
+          ? `Redirecting you to ${providerName}...`
           : 'Preparing your sign-in request...'}
       </p>
 
       {error ? (
-        <div className="mt-6 rounded-lg border border-error/30 bg-error/10 px-4 py-3 text-sm text-error">
+        <div
+          role="alert"
+          className="mt-6 rounded-lg border border-error/30 bg-error/10 px-4 py-3 text-sm text-error"
+        >
           <p>{error}</p>
           <Link
             href="/login"