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

tests/Pest.php

@@ -13,7 +13,7 @@ use Illuminate\Support\Once;
 | need to change it using the "uses()" function to bind a different classes or traits.
 |
 */
-uses(Tests\TestCase::class)->in('Feature');
+uses(Tests\TestCase::class)->in('Feature', 'v4/Feature', 'v4/Browser');
 
 /*
 |--------------------------------------------------------------------------