Build the first GPS sensor/client (browser location API)

### Prerequisites

- [x] I have read [CONTRIBUTING.md](https://github.com/EFForg/rayhunter/blob/main/CONTRIBUTING.md)

### What problem does this feature solve or what does it enhance?

Based on https://github.com/EFForg/rayhunter/pull/971.

We now should be able to use the browser's geolocation API to fill out GPS coordinates both for "static/fixed mode" and for continuously providing new location data. This would serve both as the first "official" data source for GPS and show others how to attach their own sensors.

### Proposed Solution

The input fields for lat/long in the config UI would be extended by a "fill from browser location" button, and there should be some sort of mode where if one keeps the web interface open, it continuously grabs the user's location and feeds it back to the GPS API.

### Alternatives Considered

We don't need to take any precaution as to whether the website/tab is suspended or not, we can just document that the user has to take care of that somehow. But maybe there is a way to package this as PWA or service worker that works better with Android power management.

An alternative is to extend https://codeberg.org/untitaker/rayhunter-companion or some other android app for sharing location, but that is rather platform-specific