feat: Add Excel export functionality and unit tests

Features:
- Excel export with formatted multi-sheet workbooks (Contents, Comments, Creators)
- Professional styling: blue headers, auto-width columns, borders, text wrapping
- Smart export: empty sheets automatically removed
- Support for all platforms (xhs, dy, ks, bili, wb, tieba, zhihu)

Testing:
- Added pytest framework with asyncio support
- Unit tests for Excel store functionality
- Unit tests for store factory pattern
- Shared fixtures for test data
- Test coverage for edge cases

Documentation:
- Comprehensive Excel export guide (docs/excel_export_guide.md)
- Updated README.md and README_en.md with Excel examples
- Updated config comments to include excel option

Dependencies:
- Added openpyxl>=3.1.2 for Excel support
- Added pytest>=7.4.0 and pytest-asyncio>=0.21.0 for testing

This contribution adds immediate value for users who need data analysis
capabilities and establishes a testing foundation for future development.

config/base_config.py

@@ -70,8 +70,8 @@ BROWSER_LAUNCH_TIMEOUT = 60
 # 设置为False可以保持浏览器运行，便于调试
 AUTO_CLOSE_BROWSER = True
 
-# 数据保存类型选项配置,支持四种类型：csv、db、json、sqlite, 最好保存到DB，有排重的功能。
-SAVE_DATA_OPTION = "json"  # csv or db or json or sqlite
+# 数据保存类型选项配置,支持五种类型：csv、db、json、sqlite、excel, 最好保存到DB，有排重的功能。
+SAVE_DATA_OPTION = "json"  # csv or db or json or sqlite or excel
 
 # 用户浏览器缓存的浏览器文件配置
 USER_DATA_DIR = "%s_user_data_dir"  # %s will be replaced by platform name