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.

tests/test_excel_store.py

@@ -0,0 +1,170 @@
+# -*- coding: utf-8 -*-
+"""
+Unit tests for Excel export functionality
+"""
+
+import pytest
+import asyncio
+import os
+from pathlib import Path
+import tempfile
+import shutil
+
+try:
+    import openpyxl
+    EXCEL_AVAILABLE = True
+except ImportError:
+    EXCEL_AVAILABLE = False
+
+from store.excel_store_base import ExcelStoreBase
+
+
+@pytest.mark.skipif(not EXCEL_AVAILABLE, reason="openpyxl not installed")
+class TestExcelStoreBase:
+    """Test cases for ExcelStoreBase"""
+    
+    @pytest.fixture
+    def temp_dir(self):
+        """Create temporary directory for test files"""
+        temp_path = tempfile.mkdtemp()
+        yield temp_path
+        # Cleanup
+        shutil.rmtree(temp_path, ignore_errors=True)
+    
+    @pytest.fixture
+    def excel_store(self, temp_dir, monkeypatch):
+        """Create ExcelStoreBase instance for testing"""
+        # Monkey patch data directory
+        monkeypatch.chdir(temp_dir)
+        store = ExcelStoreBase(platform="test", crawler_type="search")
+        yield store
+        # Cleanup is handled by temp_dir fixture
+    
+    def test_initialization(self, excel_store):
+        """Test Excel store initialization"""
+        assert excel_store.platform == "test"
+        assert excel_store.crawler_type == "search"
+        assert excel_store.workbook is not None
+        assert excel_store.contents_sheet is not None
+        assert excel_store.comments_sheet is not None
+        assert excel_store.creators_sheet is not None
+    
+    @pytest.mark.asyncio
+    async def test_store_content(self, excel_store):
+        """Test storing content data"""
+        content_item = {
+            "note_id": "test123",
+            "title": "Test Title",
+            "desc": "Test Description",
+            "user_id": "user456",
+            "nickname": "TestUser",
+            "liked_count": 100,
+            "comment_count": 50
+        }
+        
+        await excel_store.store_content(content_item)
+        
+        # Verify data was written
+        assert excel_store.contents_sheet.max_row == 2  # Header + 1 data row
+        assert excel_store.contents_headers_written is True
+    
+    @pytest.mark.asyncio
+    async def test_store_comment(self, excel_store):
+        """Test storing comment data"""
+        comment_item = {
+            "comment_id": "comment123",
+            "note_id": "note456",
+            "content": "Great post!",
+            "user_id": "user789",
+            "nickname": "Commenter",
+            "like_count": 10
+        }
+        
+        await excel_store.store_comment(comment_item)
+        
+        # Verify data was written
+        assert excel_store.comments_sheet.max_row == 2  # Header + 1 data row
+        assert excel_store.comments_headers_written is True
+    
+    @pytest.mark.asyncio
+    async def test_store_creator(self, excel_store):
+        """Test storing creator data"""
+        creator_item = {
+            "user_id": "creator123",
+            "nickname": "Creator Name",
+            "fans": 10000,
+            "follows": 500,
+            "interaction": 50000
+        }
+        
+        await excel_store.store_creator(creator_item)
+        
+        # Verify data was written
+        assert excel_store.creators_sheet.max_row == 2  # Header + 1 data row
+        assert excel_store.creators_headers_written is True
+    
+    @pytest.mark.asyncio
+    async def test_multiple_items(self, excel_store):
+        """Test storing multiple items"""
+        # Store multiple content items
+        for i in range(5):
+            await excel_store.store_content({
+                "note_id": f"note{i}",
+                "title": f"Title {i}",
+                "liked_count": i * 10
+            })
+        
+        # Verify all items were stored
+        assert excel_store.contents_sheet.max_row == 6  # Header + 5 data rows
+    
+    def test_flush(self, excel_store):
+        """Test flushing data to file"""
+        # Add some test data
+        asyncio.run(excel_store.store_content({
+            "note_id": "test",
+            "title": "Test"
+        }))
+        
+        # Flush to file
+        excel_store.flush()
+        
+        # Verify file was created
+        assert excel_store.filename.exists()
+        
+        # Verify file can be opened
+        wb = openpyxl.load_workbook(excel_store.filename)
+        assert "Contents" in wb.sheetnames
+        wb.close()
+    
+    def test_header_formatting(self, excel_store):
+        """Test header row formatting"""
+        asyncio.run(excel_store.store_content({"note_id": "test", "title": "Test"}))
+        
+        # Check header formatting
+        header_cell = excel_store.contents_sheet.cell(row=1, column=1)
+        assert header_cell.font.bold is True
+        assert header_cell.fill.start_color.rgb == "FF366092"
+    
+    def test_empty_sheets_removed(self, excel_store):
+        """Test that empty sheets are removed on flush"""
+        # Only add content, leave comments and creators empty
+        asyncio.run(excel_store.store_content({"note_id": "test"}))
+        
+        excel_store.flush()
+        
+        # Reload workbook
+        wb = openpyxl.load_workbook(excel_store.filename)
+        
+        # Only Contents sheet should exist
+        assert "Contents" in wb.sheetnames
+        assert "Comments" not in wb.sheetnames
+        assert "Creators" not in wb.sheetnames
+        wb.close()
+
+
+@pytest.mark.skipif(not EXCEL_AVAILABLE, reason="openpyxl not installed")
+def test_excel_import_availability():
+    """Test that openpyxl is available"""
+    assert EXCEL_AVAILABLE is True
+    import openpyxl
+    assert openpyxl is not None