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.

requirements.txt

@@ -25,4 +25,7 @@ alembic>=1.16.5
 asyncmy>=0.2.10
 sqlalchemy>=2.0.43
 motor>=3.3.0
-xhshow>=0.1.3
+xhshow>=0.1.3
+openpyxl>=3.1.2
+pytest>=7.4.0
+pytest-asyncio>=0.21.0