Log OAuth token exchange errors

app.py

@@ -88,6 +88,10 @@ def exchange_code_for_token(code: str) -> dict | None:
         resp = requests.post(HF_TOKEN_URL, data=payload, timeout=15)
         resp.raise_for_status()
         return resp.json()
+    except requests.HTTPError as http_err:
+        detail = http_err.response.text if http_err.response is not None else ""
+        st.error(f"Failed to exchange auth code: {http_err}. Response: {detail}")
+        return None
     except Exception as e:
         st.error(f"Failed to exchange auth code: {e}")
         return None