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).

routes/api.php

@@ -15,6 +15,7 @@ use App\Http\Controllers\Admin\UsersController;
 use App\Http\Controllers\AppVersionController;
 use App\Http\Controllers\Company\Auth\AuthController;
 use App\Http\Controllers\Company\Auth\ForgotPasswordController;
+use App\Http\Controllers\Company\Auth\InvitationRegistrationController;
 use App\Http\Controllers\Company\Auth\ResetPasswordController;
 use App\Http\Controllers\Company\Customer\CustomersController;
 use App\Http\Controllers\Company\Customer\CustomerStatsController;
@@ -115,6 +116,12 @@ Route::prefix('/v1')->group(function () {
         Route::post('reset/password', [ResetPasswordController::class, 'reset']);
     });
 
+    // Invitation Registration (public)
+    // ----------------------------------
+
+    Route::get('/invitations/{token}/details', [InvitationRegistrationController::class, 'details']);
+    Route::post('/auth/register-with-invitation', [InvitationRegistrationController::class, 'register']);
+
     // Countries
     // ----------------------------------