fix: improve error handling and logging in OIDC token exchange and redirect callback

frontend/src/App.tsx

@@ -166,12 +166,16 @@ async function handleRedirectCallback(): Promise<string | null> {
 
   const result = await response.json()
   const token = result.access_token || result.id_token
-  if (response.ok && token) {
-    setLocalToken(token)
-    return token
+  console.debug('OIDC callback result:', result)
+  if (!response.ok) {
+    throw new Error(result.error_description || 'OIDC callback failed')
+  }
+  if (!token) {
+    throw new Error('OIDC callback did not return an access token')
   }
 
-  throw new Error(result.error_description || 'OIDC callback failed')
+  setLocalToken(token)
+  return token
 }
 
 function App() {
@@ -207,6 +211,7 @@ function App() {
   )
 
   async function login() {
+    clearLocalToken()
     try {
       const config = await fetchOIDCConfig()
       const state = generateUUID()