refactor(flake): use pnpm2nix, add server package

[FliegendeWurst]

I made several changes:

1. Instead of pnpm.fetchDeps, use pnpm2nix. This has the benefit that we don't throw away the gigabytes-big FOD every time a tiny dependency is updated. And we don't need to keep the FOD hash updated.
2. Add a server package.
3. Optimized closure size of each package. (Build tools like pnpm should not be needed to run the package.)

Both packages were tested on NixOS 25.05.