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/steam.nix

@@ -0,0 +1,32 @@
+{ config, pkgs, lib, inputs, system, ... }:
+
+let cfg = config.sydnix.steam;
+in {
+  options.sydnix.steam = {
+    enable = lib.mkEnableOption "Steam";
+  };
+
+  config = lib.mkIf cfg.enable {
+    programs.steam = {
+      enable = true;
+      remotePlay.openFirewall = true;
+      dedicatedServer.openFirewall = true;
+      gamescopeSession.enable = true;
+      extraPackages = with pkgs; [
+        mono gtk3 gtk3-x11 libgdiplus zlib
+      ];
+    };
+
+    programs.gamemode = {
+      enable = true;
+      enableRenice = true;
+    };
+
+    # This fixes the "glXChooseVisual failed" bug, see:
+    # https://github.com/NixOS/nixpkgs/issues/47932.
+    hardware.graphics = {
+      enable = true;
+      enable32Bit = true;
+    };
+  };
+}