test: add Pest browser testing with SQLite :memory: schema

Set up end-to-end browser testing using Pest Browser Plugin + Playwright.
New v4 test suite uses SQLite :memory: database with pre-generated schema dump
(database/schema/testing-schema.sql) instead of running migrations, enabling
faster test startup.

- Add pestphp/pest-plugin-browser dependency
- Create GenerateTestingSchema command to export PostgreSQL schema to SQLite
- Add .env.testing configuration for isolated test environment
- Implement v4 test directory structure (Feature, Browser, Unit tests)
- Update Pest skill documentation with browser testing patterns, API reference,
  debugging techniques, and common pitfalls
- Configure phpunit.xml and tests/Pest.php for v4 suite
- Update package.json and docker-compose.dev.yml for testing dependencies

.env.testing

@@ -0,0 +1,15 @@
+APP_ENV=testing
+APP_KEY=base64:8VEfVNVkXQ9mH2L33WBWNMF4eQ0BWD5CTzB8mIxcl+k=
+APP_DEBUG=true
+
+DB_CONNECTION=testing
+
+CACHE_DRIVER=array
+SESSION_DRIVER=array
+QUEUE_CONNECTION=sync
+MAIL_MAILER=array
+TELESCOPE_ENABLED=false
+
+REDIS_HOST=127.0.0.1
+
+SELF_HOSTED=true