Add Playwright smoke tests for public uploads

[hhhjin]

Summary

• add Playwright config, scripts, CI wiring, and docs for browser E2E runs
• add public upload smoke coverage for initial load, preview, and anonymous publish flows
• add stable test ids and dev proxy support for public and raw markdown routes

Verification

• pnpm lint:fix
• pnpm test
• pnpm test:e2e

[gemini-code-assist]

Code Review

This pull request integrates Playwright for end-to-end testing, adding a comprehensive test suite, configuration, and data-testid attributes to several components for better selector reliability. Additionally, it refactors the TurnstileWidget to use lazy loading and updates the useUpload hook to handle widget resets via the global window object. Review feedback suggests enhancing test stability by dynamically determining the origin in E2E tests and consolidating redundant proxy rules within the Vite configuration.