refactor: add return type hints to all controller methods

Added proper return types (View, RedirectResponse, JsonResponse, Collection)
to 40+ controller methods across 16 controllers. Also added missing
imports for Illuminate response types and tightened parameter types
(e.g. InstallationController::showStep now uses int instead of mixed).

app/Http/Controllers/Shop/ShopController.php

@@ -19,7 +19,7 @@ class ShopController extends Controller
         private readonly PurchaseService $purchaseService,
     ) {}
 
-    public function __invoke(?WebsiteShopCategory $category)
+    public function __invoke(?WebsiteShopCategory $category): View
     {
         $query = WebsiteShopArticle::query()->orderBy('position');