feat(nfs): centralize nixdesk jellyfin nfs client module

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/`.

modules/hosts/r5500/configuration.nix

@@ -17,7 +17,7 @@
         ./_private/media-disk.nix
         ./_private/media-paths.nix
         ./_services/docker-media.nix
-        ./_services/nixdesk-nfs-client.nix
+        self.nixosModules.nixdeskJellyfin
         ./_services/jellyfin.nix
         ./_services/sonarr.nix
         ./_services/radarr.nix