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/Mail/CompanyInvitationMail.php

@@ -28,6 +28,13 @@ class CompanyInvitationMail extends Mailable
 
     public function content(): Content
     {
+        $token = $this->invitation->token;
+        $hasAccount = $this->invitation->user_id !== null;
+
+        $acceptUrl = $hasAccount
+            ? url("/login?invitation={$token}")
+            : url("/register?invitation={$token}");
+
         return new Content(
             markdown: 'emails.company-invitation',
             with: [
@@ -35,8 +42,9 @@ class CompanyInvitationMail extends Mailable
                 'companyName' => $this->invitation->company->name,
                 'roleName' => $this->invitation->role->title,
                 'inviterName' => $this->invitation->invitedBy->name,
-                'acceptUrl' => url("/invitations/{$this->invitation->token}/accept"),
-                'declineUrl' => url("/invitations/{$this->invitation->token}/decline"),
+                'acceptUrl' => $acceptUrl,
+                'declineUrl' => url("/invitations/{$token}/decline"),
+                'hasAccount' => $hasAccount,
             ],
         );
     }