Add end-to-end tests for circle creation and join flow

## Description
No E2E tests exist. Add Playwright tests covering the core user journey: create circle → join → contribute.

## Acceptance Criteria
- [ ] Playwright installed and configured
- [ ] Test: user can create a circle
- [ ] Test: second user can join the circle
- [ ] Test: contribution flow completes
- [ ] E2E tests run in CI against staging

**Priority:** High | **Effort:** Large