Support invitations for unregistered users

When inviting an email without an InvoiceShelf account, the email now
links to a registration page (/register?invitation={token}) instead of
login. After registering, the invitation is auto-accepted.

Backend:
- InvitationRegistrationController: public details() and register()
  endpoints. Registration validates token + email match, creates account,
  auto-accepts invitation, returns Sanctum token.
- AuthController: login now accepts optional invitation_token param to
  auto-accept invitation for existing users clicking the email link.
- CompanyInvitationMail: conditional URL based on user existence.
- Web route for /invitations/{token}/decline (email decline link).

Frontend:
- RegisterWithInvitation.vue: fetches invitation details, shows company
  name + role, registration form with pre-filled email.
- Router: /register route added.

Tests: 3 new tests (invitation details, register + accept, email mismatch).

app/Http/Controllers/Company/Auth/AuthController.php

@@ -4,7 +4,9 @@ namespace App\Http\Controllers\Company\Auth;
 
 use App\Http\Controllers\Controller;
 use App\Http\Requests\LoginRequest;
+use App\Models\CompanyInvitation;
 use App\Models\User;
+use App\Services\InvitationService;
 use Illuminate\Http\Request;
 use Illuminate\Support\Facades\Auth;
 use Illuminate\Support\Facades\Hash;
@@ -22,6 +24,17 @@ class AuthController extends Controller
             ]);
         }
 
+        // Auto-accept invitation if token is provided
+        if ($request->has('invitation_token') && $request->invitation_token) {
+            $invitation = CompanyInvitation::where('token', $request->invitation_token)
+                ->pending()
+                ->first();
+
+            if ($invitation) {
+                app(InvitationService::class)->accept($invitation, $user);
+            }
+        }
+
         return response()->json([
             'type' => 'Bearer',
             'token' => $user->createToken($request->device_name)->plainTextToken,