Playwright coverage for the dashboard First Agent wizard

## Summary

Dashboard has a multi-step First Agent wizard (`#wizard-progress`) with no browser test. Login/overview is covered; onboarding wizard is not.

## Context

- `dashboard/index.html` (wizard progress)
- `tests/browser/test_critical_paths.py` (`test_dashboard_login_and_overview`)

## Acceptance criteria

- [ ] Register/login via UI (cookies, no JWT paste)
- [ ] Open onboarding wizard and complete at least the first steps through sample call with mocks
- [ ] Assert progress marks and overview/session evidence
- [ ] `make test-browser` green

Start with dashboard/index.html to understand the #wizard-progress flow, then read tests/browser/test_critical_paths.py and its test_dashboard_login_and_overview entry point. Add browser coverage for UI registration/login, the onboarding steps through the mocked sample call, progress marks, and overview/session evidence; run make test-browser to verify completion.

Written by the indexing model from the issue text.