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.

outputs/homeConfigurations.nix

@@ -9,6 +9,7 @@ let users = builtins.readDir ../users;
 
         inputs.sops-nix.homeManagerModules.sops
         inputs.impermanence.homeManagerModules.impermanence
+        inputs.nixcord.homeManagerModules.nixcord
 
         # Directory name should always match username.
         ({ ... }: { home.username = username; })