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/ShopVoucherController.php

@@ -5,10 +5,11 @@ namespace App\Http\Controllers\Shop;
 use App\Http\Controllers\Controller;
 use App\Http\Requests\ShopVoucherFormRequest;
 use App\Models\Shop\WebsiteShopVoucher;
+use Illuminate\Http\RedirectResponse;
 
 class ShopVoucherController extends Controller
 {
-    public function __invoke(ShopVoucherFormRequest $request)
+    public function __invoke(ShopVoucherFormRequest $request): RedirectResponse
     {
         $user = $request->user();
         $voucher = WebsiteShopVoucher::where('code', $request->string('code'))->first();