Fix/webcam streamer resource leaks - #4

* fix(presets): fix edit/update preset function

Use deep clones to not store a reference to the formPreset.

* refactor(presets): refactor some code parts for better readability

The HLS and JMuxer streamers used `beforeUnmount()`, which is a Vue 3
lifecycle hook. This project runs Vue 2.7, where the hook is named
`beforeDestroy`, so neither `hls.destroy()` nor `jmuxer.destroy()` ever
ran. Every visit to a page showing one of these cameras left a decoder
and its network stream running for the rest of the session.

Hlsstreamer additionally called `play()` from `updated()`, tearing down
and rebuilding the entire Hls instance on every re-render. The `isVisible`
field — written by `v-observe-visibility` but never read — made this fire
whenever the video scrolled in or out of the viewport. Replace it with a
`@Watch('url')`, so the stream restarts only when its source actually
changes, and drop the now-unused visibility tracking.

JMuxerStream held its WebSocket in a local `const`, so nothing could
close it. Each `camSettings` change opened another socket while the old
one stayed open and kept feeding the newly created jmuxer. Keep the
socket on the instance, close it in `stop()`, and guard the handlers on
socket identity so a socket that is still closing cannot feed a newer
decoder.

Signed-off-by: Åsmund Collin <aakjaergaard@gmail.com>