Add browser and unit test suite with Pest + Playwright

Add comprehensive test coverage for core site features (content,
navigation, language, FAQ, news, petition, newsletter) using Pest
browser tests and unit tests for custom plugins. Includes test
infrastructure (Containerfile.test, compose.test.yaml), test
documentation, and test files covering petition form logic, CSV
handling, translation, date formatting, rate limiting, and map data
building.

tests/Pest.php

@@ -0,0 +1,10 @@
+<?php
+
+// Base URL for browser tests — http://app when running in compose.test.yaml
+define('BASE_URL', rtrim((string) getenv('APP_URL') ?: 'http://app', '/'));
+
+// Path to custom/ source — /app/custom in container (mounted via compose.test.yaml)
+define('CUSTOM_DIR', __DIR__ . '/../custom');
+
+// Group all Unit tests
+uses()->group('unit')->in('Unit');