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/steam.nix

@@ -0,0 +1,25 @@
+{ config, pkgs, lib, inputs, system, ... }:
+
+let cfg = config.sydnix.steam;
+in {
+  options.sydnix.steam = {
+    enable = lib.mkEnableOption "Steam";
+  };
+
+  config = lib.mkIf cfg.enable {
+    sydnix.impermanence.directories = [
+      # Steam has a number of mysterious issues when its state directories are
+      # symlinks.  Most bizarrely, I've experienced complementory issues;
+      # everything breaks when I use bind-mounts, but not
+      # symlinks.  Thanks, Valve!
+      #
+      # https://github.com/ValveSoftware/steam-for-linux/issues/10552
+      { directory = ".local/share/Steam"; method = "symlink"; }
+      { directory = ".steam"; method = "symlink"; }
+    ];
+
+    home.packages = [
+      pkgs.protonup-qt
+    ];
+  };
+}