[feat] More intuitive Live preview #2335

Describe the problem

Currently, when the live preview is connecting, we only show a half-lit lightning bolt icon in the toolbar. This is not very intuitive, as the preview panel itself remains blank. Users may not understand why the live preview isn't working or whether it's still loading.

Additionally, when the live preview encounters an error or gets disconnected, there is no clear feedback in the preview panel, which can lead to confusion.

Refer to this discussion: https://github.com/orgs/phcode-dev/discussions/2334

Describe the solution you'd like

Improve the UI by displaying a half-lit lightning bolt icon directly inside the live preview panel, along with a message such as "Live preview connecting…".

Similarly, if the live preview raises an error or gets disconnected, the panel should display a relevant icon and an informative message, so users understand the current state without having to guess.

Alternatives considered

No response

Additional context

No response