Add E2E tests for critical user workflows

[SL-Mar]

Problem

Frontend has component unit tests but zero E2E tests for critical workflows:

• Upload RTZ file → view route on map
• Calculate voyage → view results table
• Optimize route → compare fuel savings
• Configure vessel → verify persistence

Steps to fix

1. Add Cypress or Playwright E2E test suite
2. Cover 4-5 critical path workflows
3. Add to CI/CD pipeline
4. Document how to run E2E tests locally

Files

• frontend/ (new e2e test directory)
• frontend/package.json (add test dependency)