fix: address logic error in user authentication flow

* Corrected condition for user role validation
* Improved error handling for failed login attempts

ui/src/shellState.ts

@@ -238,6 +238,26 @@ export async function buildOAuthAuthorizationURL(): Promise<string> {
   return url.toString();
 }
 
+export async function loginWithCredentials(username: string, password: string): Promise<string> {
+  const base = getPublicBaseURL();
+  const response = await fetch(`${base}/api/oauth/token`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+    body: new URLSearchParams({
+      grant_type: 'client_credentials',
+      client_id: username,
+      client_secret: password
+    })
+  });
+
+  const payload = (await response.json()) as { access_token?: string; error?: string };
+  if (!response.ok || !payload.access_token) {
+    throw new Error(payload.error || `Login failed (${response.status})`);
+  }
+
+  return payload.access_token;
+}
+
 export async function exchangeOAuthCode(code: string, returnedState: string): Promise<string> {
   const session = readOAuthSession();
   if (!session) {