feat: 添加桌面版应用和心跳监控

新增功能：
- src/desktop.py: PyQt6 桌面应用，支持 Droidcam 摄像头
  - 实时视频预览 + 绿色扫描框叠加
  - 空格键快速拍照识别
  - 批量记录管理和 Excel 导出
- heartbeat.py: 服务心跳监控，自动重启 Streamlit
- requirements-desktop.txt: 桌面版专用依赖

Web 版优化：
- src/app.py: 自定义摄像头组件，扫描框叠加到视频流

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

heartbeat.py

@@ -0,0 +1,53 @@
+#!/usr/bin/env python3
+"""心跳程序 - 保持服务活跃"""
+import sys
+import time
+import subprocess
+import requests
+from datetime import datetime
+
+# 禁用输出缓冲
+sys.stdout.reconfigure(line_buffering=True)
+
+
+def log(msg):
+    print(f"[{datetime.now():%Y-%m-%d %H:%M:%S}] {msg}", flush=True)
+
+
+def check_streamlit():
+    """检查 Streamlit 服务"""
+    try:
+        r = requests.get("http://localhost:8501", timeout=5)
+        return r.status_code == 200
+    except:
+        return False
+
+
+def restart_streamlit():
+    """重启 Streamlit"""
+    subprocess.run(["pkill", "-f", "streamlit run"], capture_output=True)
+    time.sleep(2)
+    subprocess.Popen(
+        ["streamlit", "run", "src/app.py", "--server.port", "8501", "--server.address", "0.0.0.0"],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+    )
+    print(f"[{datetime.now():%H:%M:%S}] Streamlit 已重启")
+
+
+def main():
+    log("心跳程序启动")
+
+    while True:
+        if not check_streamlit():
+            log("Streamlit 无响应，正在重启...")
+            restart_streamlit()
+            time.sleep(10)
+        else:
+            log("✓ 服务正常")
+
+        time.sleep(60)  # 每分钟检查一次
+
+
+if __name__ == "__main__":
+    main()