feat: Add CI testing infrastructure with act_runner support

- Created scripts/test-ci-locally.sh to test Gitea Actions workflows locally using act_runner
- Created docker-compose.ci.yml for containerized CI test environment
- Updated .gitea/workflows/magnitude.yml to use docker-compose for CI
- Added scripts/README.md documenting the CI testing approach
- Created reusable test helpers in tests/playwright/

This allows developers to run the exact same workflow that CI runs, locally,
making it much easier to debug CI failures without push cycles.

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

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

tests/playwright/seed.spec.ts

@@ -0,0 +1,7 @@
+import { test, expect } from '@playwright/test';
+
+test.describe('Test group', () => {
+  test('seed', async ({ page }) => {
+    // generate code here.
+  });
+});