feat: add PHPUnit test infrastructure and Router tests

- Add composer.json with PHPUnit 11 and PSR-4 autoloading
- Add phpunit.xml configuration
- Rename app/core/ → app/Core/ and app/controllers/ → app/Controllers/ (PSR-4)
- Add 18 unit tests for App\Core\Router (31 assertions)
  - addRoute(): default action, custom action, overwrite
  - dispatch(): URL normalization, direct match, legacy redirects
  - dispatch(): 404 handling, controller/action not found exceptions
  - TestableRouter subclass to intercept side-effects
- Update .gitignore (vendor/, .phpunit.cache/)

composer.json

@@ -0,0 +1,16 @@
+{
+    "name": "greggy/landingpage-haus-schleusingen",
+    "autoload": {
+        "psr-4": {
+            "App\\": "app/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Tests\\": "tests/"
+        }
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^11.0"
+    }
+}