feat: added nix support

[LibereCode]

Summary

This doesn't really changes anything, it's just a small wrapper for nix,
where it will create a shell-script that runs the AppImage.

I have tested it locally with:

## Build ok
nix build .
# or you can also use `nix build .#seanime-denshi`

## Runs and opens the app
nix run .

I have used no AI.

Checklist

• I have read CONTRIBUTING.md
• I set up Seanime locally.
• I understand the code I changed.
• I tested the change locally.
• I wrote this PR description myself.
• I disclosed any AI assistance.
• I can explain and maintain this change.

[LibereCode]

Note, it is just a wrapper for seanime-denshi; there already exist a nixpkg for seanime (the server)