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/14900k/_private/media-disk.nix

@@ -1,9 +1,5 @@
 # Extra local disks. Declared here, not in hardware.nix (hardware.nix is generated).
 { config, lib, ... }:
-let
-  # Stable UID so NTFS `uid=` matches `users.users.olivier` (override if your account is not 1000).
-  olivierUid = config.users.users.olivier.uid or 1000;
-in
 {
   users.users.olivier.uid = lib.mkDefault 1000;
   # LABEL="MediaLibrary" (btrfs on sda1 by UUID). No subvol=@ — this disk has no @ subvolume.
@@ -33,15 +29,4 @@ in
   # LABEL="Deep Storage Unit". Owner olivier, group nfsmedia (990) so:
   # - local logins write as user 1000 (owner rwx);
   # - NFS (all_squash → uid/gid 990) matches group 990 → rwx (see jellyfin-nfs-export).
-  #fileSystems."/mnt/test" = {
-  #  device = "/dev/disk/by-uuid/BC12E55E12E51DE0";
-  #  fsType = "ntfs-3g";
-  #  options = [
-  #    "rw"
-  #    "force"
-  #    "uid=${toString olivierUid}"
-  #    "gid=990"
-  #    "umask=0002"
-  #  ];
-  #};
 }