Make opening the web browser at startup optional somehow

I have Sunshine set to autostart in KDE when my system boots, and it always opens a web browser on `localhost:47990`, which I don't want. There doesn't appear to be a way to opt out of this behavior.

See also https://github.com/flathub/dev.lizardbyte.app.Sunshine/pull/87, although that PR alone won't suffice as `xdg-open` is called regardless. We could perhaps have a custom CLI argument in the launcher script for this, such as `--no-browser`. It can then be documented in the README.

Start by inspecting the launcher script and the existing xdg-open call mentioned in the issue, then review the README for current startup options. Determine how a --no-browser-style option would be passed through the launcher, and confirm that startup can avoid opening localhost:47990 while the default behavior remains documented.

Written by the indexing model from the issue text.