refactor: migrate configuration to .env file for better environment management

- Replace hardcoded file paths with environment variables in extract_vba.py and import_vba.py
- Add .env file for local configuration (excluded from git via .gitignore)
- Add python-dotenv dependency for environment variable loading
- Maintain fallback to interactive mode when environment variables are not set

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.gitignore

@@ -5,6 +5,7 @@ dist
 log
 *.spec
 temp
+.env
 
 # Claude 临时文件
 .claude/