redirect training subprocess stderr to log file for debugging

app.py

@@ -410,10 +410,11 @@ finally:
         with open(script_path, "w") as f:
             f.write(train_script)
 
+        train_stderr = os.path.join(output_dir, "train_stderr.log")
         subprocess.Popen(
-            ["python3", script_path],
-            stdout=open("/dev/null", "w"),
-            stderr=open("/dev/null", "w"),
+            ["python3", "-u", script_path],
+            stdout=open(TRAIN_LOG, "a"),
+            stderr=open(train_stderr, "w"),
             start_new_session=True,
         )
 
@@ -423,10 +424,14 @@ finally:
                 f"Inference will be unavailable until training completes (ace-server stopped).")
 
     def check_train_log():
-        if not os.path.exists(TRAIN_LOG):
-            return "No training log found."
-        with open(TRAIN_LOG) as f:
-            return f.read() or "Log is empty."
+        parts = []
+        if os.path.exists(TRAIN_LOG):
+            parts.append(open(TRAIN_LOG).read())
+        stderr_log = os.path.join(ADAPTER_DIR, "test-lora", "train_stderr.log")
+        if os.path.exists(stderr_log) and os.path.getsize(stderr_log) > 0:
+            stderr = open(stderr_log).read()[-2000:]
+            parts.append(f"\n--- stderr (last 2000 chars) ---\n{stderr}")
+        return "\n".join(parts) if parts else "No training log found."
 
     # -- Build UI --
     CSS = """