feat: add HTML report screenshot generation via Playwright

- Add html_report.py module for Playwright-based screenshot generation
- Add generate_html_report() method to SimpleRotationStrategy
- Modify backtest_viewer.html to use window-scoped variables for external injection
- Inject monthly/yearly returns table into screenshot
- Auto-generate HTML report in __main__ after export_results()

Output: simple_rotation_html_report.png with ranking table + monthly returns

.gitignore

@@ -199,3 +199,9 @@ zhihu-articles/
 
 # Results directory (test outputs, charts, etc.)
 results/
+
+# Node.js (local testing only)
+node_modules/
+package.json
+package-lock.json
+rotation/test_screenshot.mjs