feat: show identity match hint on registration success page

When a pending identity match is detected after volunteer registration,
the API now returns has_existing_account in the response. The success
page shows a login suggestion card so the volunteer can link their
registration to their existing account.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Http/Controllers/Api/V1/VolunteerRegistrationController.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api\V1;
+
+use App\Enums\IdentityMatchStatus;
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Api\V1\VolunteerRegistrationRequest;
+use App\Http\Resources\Api\V1\PersonResource;
+use App\Models\Event;
+use App\Models\PersonIdentityMatch;
+use App\Services\VolunteerRegistrationService;
+use Illuminate\Http\JsonResponse;
+
+final class VolunteerRegistrationController extends Controller
+{
+    public function __construct(
+        private readonly VolunteerRegistrationService $registrationService,
+    ) {}
+
+    public function __invoke(VolunteerRegistrationRequest $request, Event $event): JsonResponse
+    {
+        $user = auth('sanctum')->user();
+
+        $person = $this->registrationService->register(
+            $event,
+            $request->validated(),
+            $user
+        );
+
+        $person->load('crowdType');
+
+        $hasExistingAccount = PersonIdentityMatch::where('person_id', $person->id)
+            ->where('status', IdentityMatchStatus::PENDING)
+            ->exists();
+
+        $responseData = [
+            'person' => new PersonResource($person),
+            'has_existing_account' => $hasExistingAccount,
+        ];
+
+        if ($person->wasRecentlyCreated) {
+            return $this->created($responseData);
+        }
+
+        return $this->success($responseData);
+    }
+}