Fix Alpine state reference and remove unused property in upgrade modal

- Fix incorrect Alpine state reference: Changed `this.$wire.showProgress` to `this.showProgress` in upgrade.blade.php:155
- Remove unused `$showProgress` property from Upgrade.php Livewire component
- The backend property was never set or used; all progress tracking is handled by Alpine state
- This fixes potential race conditions where the guard condition was not working as intended

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Livewire/Upgrade.php

@@ -8,8 +8,6 @@ use Livewire\Component;
 
 class Upgrade extends Component
 {
-    public bool $showProgress = false;
-
     public bool $updateInProgress = false;
 
     public bool $isUpgradeAvailable = false;