Run flatpaked emacs in the background

I think there should be a way to run flatpaked emacs in the background and use `emacsclient` to connect.

On modern distributions, the traditional way is to create a systemd service, enable it for user and run the application on session startup, see . Then just add emacsclient that creates a new frame to autostart, create `emacsclient.desktop` file and open each file in the same frame.

Flatpak now allows to run applications in the backgound and use autostart,
see .

In my opinion, using just one frame with `emacsclient` provides better user experience than using a newly-created frame for each buffer. Since flatpaked emacs is used in desktop environments, I think this way should be available or even take precedence.

As I see it, `org.gnu.emacs.client.desktop` file should definitely be provided by the package. Also there is need to investigate how configure the package to run emacs in the background. The corresponding instruction should be added to `emacs-first-run.txt`.

Start by reviewing the Flatpak background and autostart requirements described in the issue, along with how org.gnu.emacs.client.desktop would be provided. Then inspect emacs-first-run.txt and the package configuration relevant to running Emacs in the background. Done means the client desktop entry and background setup are available, with instructions documented for using emacsclient.

Written by the indexing model from the issue text.