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/conftest.py

@@ -0,0 +1,81 @@
+# -*- coding: utf-8 -*-
+"""
+Pytest configuration and shared fixtures
+"""
+
+import pytest
+import sys
+from pathlib import Path
+
+# Add project root to Python path
+project_root = Path(__file__).parent.parent
+sys.path.insert(0, str(project_root))
+
+
+@pytest.fixture(scope="session")
+def project_root_path():
+    """Return project root path"""
+    return project_root
+
+
+@pytest.fixture
+def sample_xhs_note():
+    """Sample Xiaohongshu note data for testing"""
+    return {
+        "note_id": "test_note_123",
+        "type": "normal",
+        "title": "测试标题 Test Title",
+        "desc": "这是一个测试描述 This is a test description",
+        "video_url": "",
+        "time": 1700000000,
+        "last_update_time": 1700000000,
+        "user_id": "user_123",
+        "nickname": "测试用户",
+        "avatar": "https://example.com/avatar.jpg",
+        "liked_count": 100,
+        "collected_count": 50,
+        "comment_count": 25,
+        "share_count": 10,
+        "ip_location": "上海",
+        "image_list": "https://example.com/img1.jpg,https://example.com/img2.jpg",
+        "tag_list": "测试,编程,Python",
+        "note_url": "https://www.xiaohongshu.com/explore/test_note_123",
+        "source_keyword": "测试关键词",
+        "xsec_token": "test_token_123"
+    }
+
+
+@pytest.fixture
+def sample_xhs_comment():
+    """Sample Xiaohongshu comment data for testing"""
+    return {
+        "comment_id": "comment_123",
+        "create_time": 1700000000,
+        "ip_location": "北京",
+        "note_id": "test_note_123",
+        "content": "这是一条测试评论 This is a test comment",
+        "user_id": "user_456",
+        "nickname": "评论用户",
+        "avatar": "https://example.com/avatar2.jpg",
+        "sub_comment_count": 5,
+        "pictures": "",
+        "parent_comment_id": 0,
+        "like_count": 15
+    }
+
+
+@pytest.fixture
+def sample_xhs_creator():
+    """Sample Xiaohongshu creator data for testing"""
+    return {
+        "user_id": "creator_123",
+        "nickname": "创作者名称",
+        "gender": "女",
+        "avatar": "https://example.com/creator_avatar.jpg",
+        "desc": "这是创作者简介",
+        "ip_location": "广州",
+        "follows": 500,
+        "fans": 10000,
+        "interaction": 50000,
+        "tag_list": '{"profession": "设计师", "interest": "摄影"}'
+    }