feat(payment): implement OxaPay provider and non-recurring subscription sync

- Add comprehensive OxaPay payment provider with invoice creation, webhook processing, and subscription status sync
  - Implement conditional payload fields (to_currency, callback_url) based on configuration
  - Create universal sync command for all non-recurring payment providers
  - Add subscription model fields for payment tracking
  - Implement proper status mapping between OxaPay and Laravel subscription states
  - Add webhook signature validation using HMAC SHA512

resources/views/livewire/settings/billing.blade.php

@@ -116,7 +116,7 @@
                                 </flux:button>
                             @endif
 
-                            @if($latestActiveSubscription->isActive() && in_array($latestActiveSubscription->provider, ['activation_key']))
+                            @if($latestActiveSubscription->isActive() && in_array($latestActiveSubscription->provider, ['activation_key', 'oxapay']))
                                 <flux:button
                                     wire:click="confirmCancelSubscription({{ $latestActiveSubscription->id }})"
                                     variant="danger"
@@ -266,4 +266,4 @@
             </div>
         </flux:modal>
     </x-settings.layout>
-</section>
+</section>