[feat] Notification onclick event

### Describe the problem

I am running my Tauri app in the tray and I only want to show the UI when the user responds to a notification. I can't seem to subscribe to any notification onclick event.

### Describe the solution you'd like

I would like to be able to subscribe to a click event, like so:

```
let n = new Notification('title', { body: 'some text' });
n.onclick = function() { console.log('click happened') };
```

### Alternatives considered

_No response_

### Additional context

_No response_

No implementation files or tests are named in the issue. Start by reviewing the Tauri notification plugin's existing API and the 24-comment discussion to determine the supported click-event design and platform scope. Done means applications can subscribe to a notification click and receive the event reliably.

Written by the indexing model from the issue text.