Allow relative urls for monitor widget

### Description

Right now, I host many services on the same domain, under different subpaths, for example `mydomain.com/forgejo` and `mydomain.com/radicale`. It would be nice if the monitor widget supported relative urls, so that instead of providing the complete url with the same domain each time, I can simply provide the relative url. For example, instead of the following:
```yaml
- type: monitor
title: Services
sites:
- title: Forgejo
url: https://mydomain.com/forgejo
```
I wish to be able to write:
```yaml
- type: monitor
title: Services
sites:
- title: Forgejo
url: /forgejo
```

No file or test is named. Start by tracing how the monitor widget reads and resolves each site's URL, then check existing monitor configuration and URL-handling tests if present. Done means a site configured with a relative URL such as /forgejo is monitored successfully alongside absolute URLs.

Written by the indexing model from the issue text.