Move the nixdesk jellyfin NFS client configuration from individual
host services to a centralized NixOS module in `modules/system/nfs/`.
This change:
- Creates `modules/system/nfs/default.nix` containing the `nixdeskJellyfin`
flake module.
- Removes redundant NFS client definitions from `modules/hosts/nix-server/`
and `modules/hosts/r5500/`.
- Updates `modules/hosts/r5500/configuration.nix` to use the new
centralized module.
- Cleans up unused local disk configuration in `modules/hosts/14900k/`.
