Improve configuration-page user interaction with dismissible alerts

[Copilot]

The issue requested improved user experience/user interaction. This change focuses on reducing UI friction in configuration workflows by letting users explicitly clear transient feedback messages.

• Config page alert interaction

  • Added dismiss controls to both error and success alerts in ConfigPage.
  • Alerts are now user-controlled instead of passively occupying screen space after actions.

• Alert layout/styling updates

  • Added .alert-message { flex: 1; } so message text and actions align cleanly.
  • Added reusable .alert-close styles (transparent button, inherited color, hover opacity) to keep visuals consistent with existing alert themes.

<button
    class="alert-close"
    type="button"
    aria-label="Dismiss error message"
    on:click=move |_| set_error_message.set(None)
>
    "✕"
</button>