Implement Tests and QA Validation

## Overview
Implement comprehensive testing and QA validation for the application.

## Acceptance Criteria
- [ ] Backend unit: runner ownership enforcement
- [ ] Backend integration: register → login → me → create runner (happy path)
- [ ] Frontend tests: route guard (unauth → /login), post-login redirect, runner form validation
- [ ] Manual QA pass (see template below)
- [ ] Tests pass locally/CI; manual QA finds no blockers

## Test Notes
- Test runner ownership enforcement on backend
- Test complete user flow from registration to runner creation
- Test frontend route guards and redirects
- Test form validation and error handling

## Implementation Notes
- Use Jest for frontend testing
- Use xUnit for backend testing
- Implement integration tests for API endpoints
- Create manual QA checklist for user flows

Locate the backend runner ownership and API integration entry points, then the frontend route-guard and runner-form code. Use xUnit for backend tests and Jest for frontend tests; finish by exercising registration through runner creation and completing the manual QA checklist with no blockers.

Written by the indexing model from the issue text.