feat: add team invitation acceptance flow with email link routing

Implement end-to-end invitation acceptance: neutral entry route validates
token and routes to register (new users), login (existing users), or
auto-accepts (authenticated users). Handles 2FA token survival via
session, email case-insensitive matching, and dedicated error pages.

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

app/Http/Responses/RegisterResponse.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Responses;
+
+use Illuminate\Http\JsonResponse;
+use Laravel\Fortify\Contracts\RegisterResponse as RegisterResponseContract;
+
+class RegisterResponse implements RegisterResponseContract
+{
+    /**
+     * Create an HTTP response that represents the object.
+     *
+     * @param  \Illuminate\Http\Request  $request
+     * @return \Symfony\Component\HttpFoundation\Response
+     */
+    public function toResponse($request)
+    {
+        if ($request->session()->pull('invitation_accepted')) {
+            return redirect()->intended('/dashboard');
+        }
+
+        return $request->wantsJson()
+            ? new JsonResponse('', 201)
+            : redirect()->intended(config('fortify.home'));
+    }
+}