feat: Add host sydpc

My, this is a lot TwT.  Much work was batched as part of the transition from
guix-rebound to nixos-testbed/sydpc.

- Discord/Vesktop module & config.
- Syncthing setup.
- Assorted Emacs changes.
- Waybar config.
- Niri config.
- Steam config.
- Some MPD.
- Stylix config.
- Files/Impermanence things.
- Enable Ghostty.
- God knows what else.

modules/home/users/crumb/mpd.nix

@@ -30,7 +30,10 @@ in {
 
     # mpdscribble uses our password files, so it is imperative that the service
     # runs only after said password files are brought into existence. }:)
-    systemd.user.services.mpdscribble.Unit.After = [ "sops-nix.service" ];
+    systemd.user.services.mpdscribble.Unit = {
+      After = [ "sops-nix.service" ];
+      WantedBy = [ "mpd.service" ];
+    };
   };
 }