Auth: convert auth views and verification email to Nova layout

app/Http/Controllers/Api/UsernameAvailabilityController.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\UsernameRequest;
+use App\Models\User;
+use App\Support\UsernamePolicy;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+
+final class UsernameAvailabilityController extends Controller
+{
+    public function __invoke(Request $request): JsonResponse
+    {
+        $candidate = UsernamePolicy::normalize((string) $request->query('username', ''));
+
+        $validator = validator(
+            ['username' => $candidate],
+            ['username' => UsernameRequest::formatRules()]
+        );
+
+        if ($validator->fails()) {
+            return response()->json([
+                'available' => false,
+                'normalized' => $candidate,
+                'errors' => $validator->errors()->toArray(),
+            ], 422);
+        }
+
+        $ignoreUserId = $request->user()?->id;
+        $exists = User::query()
+            ->whereRaw('LOWER(username) = ?', [$candidate])
+            ->when($ignoreUserId !== null, fn ($q) => $q->where('id', '!=', (int) $ignoreUserId))
+            ->exists();
+
+        return response()->json([
+            'available' => ! $exists,
+            'normalized' => $candidate,
+        ]);
+    }
+}