feat: add VAT settings page and controller

app/Http/Controllers/Settings/VatSettingController.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Http\Controllers\Settings;
+
+use App\Http\Controllers\Controller;
+use App\Models\Setting;
+use Illuminate\Http\Request;
+
+class VatSettingController extends Controller
+{
+    public function index()
+    {
+        $vatRate = Setting::get('vat_rate', '0');
+        return view('settings.vat', compact('vatRate'));
+    }
+
+    public function update(Request $request)
+    {
+        $validated = $request->validate([
+            'vat_rate' => ['required', 'numeric', 'min:0', 'max:100'],
+        ]);
+
+        Setting::set('vat_rate', (string) $validated['vat_rate']);
+
+        return response()->json(['message' => 'VAT rate saved.', 'vat_rate' => $validated['vat_rate']]);
+    }
+}