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.

store/xhs/__init__.py

@@ -37,13 +37,14 @@ class XhsStoreFactory:
         "json": XhsJsonStoreImplement,
         "sqlite": XhsSqliteStoreImplement,
         "mongodb": XhsMongoStoreImplement,
+        "excel": XhsExcelStoreImplement,
     }
 
     @staticmethod
     def create_store() -> AbstractStore:
         store_class = XhsStoreFactory.STORES.get(config.SAVE_DATA_OPTION)
         if not store_class:
-            raise ValueError("[XhsStoreFactory.create_store] Invalid save option only supported csv or db or json or sqlite or mongodb ...")
+            raise ValueError("[XhsStoreFactory.create_store] Invalid save option only supported csv or db or json or sqlite or mongodb or excel ...")
         return store_class()