Handle grayjay://video and grayjay://channel protocols

Making e.g. a `~/.local/share/applications/grayjay-protocol-handler.desktop` with:
```ini
[Desktop Entry]
Name=Grayjay protocol handler
Exec=/usr/bin/flatpak run --branch=stable --command=Grayjay app.grayjay.Grayjay %u
Icon=app.grayjay.Grayjay
Type=Application
Terminal=false
MimeType=x-scheme-handler/grayjay;
X-Flatpak=app.grayjay.Grayjay
```
(and running `xdg-mime default grayjay-protocol-handler.desktop x-scheme-handler/grayjay`? maybe not needed) allows the Grayjay flatpak to open upon e.g. `xdg-open grayjay://video/https://www.youtube.com/watch?v=EnZrv37u66c` but it just opens the home page (in a new instance fwiw). I believe the `MimeType` and `Exec=... %u` tweaks could be just added to https://gitlab.futo.org/videostreaming/Grayjay.Desktop/-/blob/master/app.grayjay.Grayjay.desktop alternatively as well.

In the Android app these seem to be handled in https://gitlab.futo.org/videostreaming/grayjay/-/blob/de39451f/app/src/main/java/com/futo/platformplayer/activities/MainActivity.kt#L797-803 but I have no idea where in the desktop app codebase such logic would fit nor could I contribute it regardless since https://gitlab.futo.org/videostreaming/Grayjay.Desktop/-/issues and https://gitlab.futo.org/videostreaming/Grayjay.Desktop/-/merge_requests are private with no way for "an outsider" to make an account/otherwise login on the FUTO gitlab instance either.

The end goal would be to get e.g. https://greasyfork.org/en/scripts/521170-redirect-youtube-to-grayjay/code working just like it already does on Android; maybe I even missed something that can already accomplish this on the desktop app somehow else instead.