Set up end-to-end testing with Playwright

[Smartdevs17]

Description

No E2E testing exists for critical user flows. Set up Playwright test suite covering payment flow, onboarding, and dispute resolution.

Acceptance Criteria

• Playwright configuration with CI integration
• Payment flow E2E test (create -> fund -> confirm)
• Onboarding flow E2E test with document upload
• Dispute resolution flow E2E test
• Visual regression tests for key pages
• Test data fixtures and cleanup

Technical Scope

• frontend/e2e/
• .github/workflows/e2e.yml
• Edge: flaky test handling, test isolation, headless mode recording