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.

main.py

@@ -84,6 +84,18 @@ async def main():
     crawler = CrawlerFactory.create_crawler(platform=config.PLATFORM)
     await crawler.start()
 
+    # Flush Excel data if using Excel export
+    if config.SAVE_DATA_OPTION == "excel":
+        try:
+            # Get the store instance and flush data
+            from store.xhs import XhsStoreFactory
+            store = XhsStoreFactory.create_store()
+            if hasattr(store, 'flush'):
+                store.flush()
+                print(f"[Main] Excel file saved successfully")
+        except Exception as e:
+            print(f"Error flushing Excel data: {e}")
+
     # Generate wordcloud after crawling is complete
     # Only for JSON save mode
     if config.SAVE_DATA_OPTION == "json" and config.ENABLE_GET_WORDCLOUD: