feat: 添加桌面程序打包支持

- 添加 PyInstaller 打包脚本 build_exe.py
- 更新 requirements.txt 添加 PyQt6 和 opencv-python
- .gitignore 添加 PyInstaller 打包产物忽略规则

使用方法:
  pip install pyinstaller
  python build_exe.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build_exe.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""
+打包脚本 - 将桌面程序打包成独立可执行文件
+使用方法: pip install pyinstaller && python build_exe.py
+"""
+import subprocess
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).parent
+
+
+def build():
+    """使用 PyInstaller 打包"""
+
+    # PyInstaller 命令
+    cmd = [
+        "pyinstaller",
+        "--name=信封信息提取系统",
+        "--onefile",              # 打包成单个 exe
+        "--windowed",             # 无控制台窗口
+        "--clean",                # 清理缓存
+        "--noconfirm",            # 覆盖输出目录不询问
+        "--add-data=src;src",    # 包含源码目录
+        "--hidden-import=PaddleOCR",
+        "--hidden-import=paddleocr",
+        "--hidden-import=cv2",
+        "--hidden-import=PyQt6",
+        "--collect-all=PaddleOCR",
+        "--collect-all=paddleocr",
+        "src/desktop.py",
+    ]
+
+    print("正在打包，请稍候...")
+    print(f"工作目录: {PROJECT_ROOT}")
+    print(f"输出目录: {PROJECT_ROOT / 'dist'}")
+    print("-" * 50)
+
+    subprocess.run(cmd, check=True, cwd=PROJECT_ROOT)
+
+    print("-" * 50)
+    print("打包完成！")
+    print(f"可执行文件: {PROJECT_ROOT / 'dist' / '信封信息提取系统.exe'}")
+
+
+if __name__ == "__main__":
+    build()