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/nixos/niri.nix

@@ -0,0 +1,13 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.sydnix.niri;
+in {
+  options.sydnix.niri.enable =
+    lib.mkEnableOption "Niri";
+
+  config = lib.mkIf cfg.enable {
+    programs.niri.enable = true;
+  };
+}
+