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/nixosConfigurations.nix

@@ -34,6 +34,8 @@ let
         inputs.sops-nix.nixosModules.sops
         inputs.impermanence.nixosModules.impermanence
         inputs.copyparty.nixosModules.default
+        inputs.niri.nixosModules.niri
+        inputs.stylix.nixosModules.stylix
 
         # Directory name should always match host name.
         ({ ... }: { networking.hostName = hostName; })