feat: 添加轻量打包和OCR自动安装功能

- 添加Windows打包脚本 build.bat
- 更新打包文档 BUILD.md（轻量版方案）
- OCR模块：添加首次运行时自动安装PaddleOCR的功能
- 主窗口：添加OCR安装检测和提示逻辑
- 创建应用入口 src/main.py

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

src/gui/main_window.py

@@ -19,8 +19,9 @@ from PyQt6.QtWidgets import (
     QFileDialog,
     QMessageBox
 )
-from PyQt6.QtCore import Qt, QSize, pyqtSignal
-from PyQt6.QtGui import QIcon, QShortcut, QKeySequence
+from PyQt6.QtCore import Qt, QSize, pyqtSignal, QThread, QTimer
+from PyQt6.QtGui import QIcon, QShortcut, QKeySequence, QMessageBox
+from src.core.ocr import ensure_paddleocr
 
 from src.gui.styles import ThemeStyles
 from src.gui.widgets import (