feat: Add comprehensive testing infrastructure

Implements robust testing setup with Playwright global auth, reusable test
helpers, Docker support, and CI/CD integration with Gitea Actions.

## Changes

### Playwright Setup
- Add global auth setup with storage state reuse (tests/playwright/auth.setup.ts)
- Fix auth setup to clear existing state before fresh login
- Create reusable performOAuthLogin helper (tests/playwright/helpers.ts)
- Configure dotenv loading for environment variables in playwright.config.ts

### Magnitude Configuration
- Update to use Claude Sonnet 4.5 (claude-sonnet-4-5-20250514)
- Create reusable loginFlow helper (tests/magnitude/helpers.ts)
- Fix smoke test to check login page instead of non-existent homepage

### Docker Support
- Add Dockerfile.playwright with non-root user (pwuser) for security
- Uses official Playwright Docker image (mcr.microsoft.com/playwright:v1.49.1-noble)
- Provides consistent testing environment across users and CI/CD

### CI/CD Integration
- Add Gitea Actions workflow (.gitea/workflows/magnitude.yml)
- Runs Magnitude tests on every push and PR
- Starts SurrealDB and Next.js dev server automatically
- Uploads test results as artifacts (30-day retention)

### Documentation
- Add comprehensive testing setup docs to AGENTS.md:
  - Playwright Docker setup instructions
  - CI/CD with Gitea Actions
  - Testing framework separation (Playwright vs Magnitude)
  - Required secrets for CI/CD

### Testing Best Practices
- Separate Playwright (manual + global auth) from Magnitude (automated E2E)
- Reusable helpers reduce code duplication
- Both frameworks work independently

## Testing

- ✅ Playwright auth setup test passes (5.6s)
- ✅ Magnitude smoke test passes
- ✅ OAuth flow works correctly with helper function

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

magnitude.config.ts

@@ -6,4 +6,6 @@ export default {
   tests: 'tests/magnitude/**/*.mag.ts',
   // Run tests in headless mode to avoid window focus issues
   headless: true,
+  // Use Claude Sonnet 4.5 for best performance
+  model: 'anthropic:claude-sonnet-4-5-20250514',
 };