feat: add pricing display to Hetzner server creation button

Display the monthly cost on the "Buy & Create Server" button
to give users clear visibility of the price before purchasing.

- Add computed property to calculate selected server's monthly price
- Update button text to show price dynamically (e.g., "€12.99/mo")
- Add tests for price formatting and edge cases
- Price updates reactively when user changes server type

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

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

resources/views/livewire/server/new/by-hetzner.blade.php

@@ -162,7 +162,7 @@
                         </x-forms.button>
                         <x-forms.button isHighlighted canGate="create" :canResource="App\Models\Server::class" type="submit"
                             :disabled="!$private_key_id">
-                            Buy & Create Server
+                            Buy & Create Server{{ $this->selectedServerPrice ? ' (' . $this->selectedServerPrice . '/mo)' : '' }}
                         </x-forms.button>
                     </div>
                 </form>