Improve JUC500 device discovery and Windows doctor diagnostics

Widen libusb DLL search, copy DLL into venv on setup, match sibling
PIDs, and add doctor command with PnP VID_0711 hints for WinUSB.

Co-authored-by: Cursor <cursoragent@cursor.com>

juc500_xfer/gui_server.py

@@ -16,7 +16,7 @@ from pathlib import Path
 from typing import Any, Optional
 
 from .protocol import Session
-from .usb_link import Juc500Link, list_devices
+from .usb_link import Juc500Link, list_devices, doctor
 
 DEFAULT_GUI_PORT = 8765
 
@@ -398,8 +398,23 @@ class Handler(BaseHTTPRequestHandler):
 
     def _api_get(self, path: str, qs: dict[str, list[str]]) -> None:
         if path == "/api/status":
-            devices = list_devices()
+            try:
+                devices = list_devices()
+                list_err = None
+            except Exception as exc:  # noqa: BLE001
+                devices = []
+                list_err = str(exc)
             cable = devices[0] if devices else None
+            if devices:
+                auto = STATE.auto_status if _session_alive() else "케이블 감지됨 — 연결을 누르세요"
+            elif list_err:
+                auto = f"USB 오류: {list_err}"
+            else:
+                auto = (
+                    "JUC500 미감지 — python -m juc500_xfer doctor 로 진단하세요"
+                    if not STATE.connecting
+                    else STATE.auto_status
+                )
             self._json(
                 200,
                 {
@@ -417,13 +432,16 @@ class Handler(BaseHTTPRequestHandler):
                     "root": STATE.peer_name or "JUC500",
                     "remote_start": "",
                     "remote_receive": "",
-                    "auto_status": STATE.auto_status,
+                    "auto_status": auto if not _session_alive() else STATE.auto_status,
                     "progress": STATE.last_progress,
                     "device": {
                         "serial": cable.serial if cable else None,
                         "product": cable.product if cable else None,
                         "count": len(devices),
+                        "vid": f"{cable.vid:04x}" if cable else None,
+                        "pid": f"{cable.pid:04x}" if cable else None,
                     },
+                    "usb_error": list_err,
                     "inputshare": {
                         "supported": False,
                         "enabled": False,
@@ -434,6 +452,10 @@ class Handler(BaseHTTPRequestHandler):
             )
             return
 
+        if path == "/api/doctor":
+            self._json(200, doctor().to_dict())
+            return
+
         if path == "/api/settings/ports":
             # Reuse settings modal: expose local_root / serial as "ports"-like rows
             self._json(