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/discord.nix

@@ -0,0 +1,63 @@
+{ config, lib, pkgs, ... }:
+
+let cfg = config.sydnix.discord;
+in {
+  options.sydnix.discord = {
+    enable = lib.mkEnableOption "Discord";
+    fixWayland = lib.mkOption {
+      description = ''
+        Fix flickering on Wayland.
+      '';
+      type = lib.types.bool;
+      default = false;
+    };
+  };
+
+  config = lib.mkIf cfg.enable
+    (let
+      # Fixes flickering and platform detection on Wayland.
+      vesktop-wayland-wrapper =
+        pkgs.symlinkJoin {
+          name = "vesktop-wayland-wrapper";
+          paths = [ pkgs.callPackage pkgs.vesktop ];
+          buildInputs = [ pkgs.makeWrapper ];
+          postBuild = ''
+            wrapProgram $out/bin/vesktop \
+              --add-flags "--disable-gpu-compositing"
+              # --add-flags "--disable-gpu-compositing --ozone-platform-hint=auto"
+          '';
+        };
+    in {
+      sydnix.impermanence.directories =
+        let xdg-config-home = 
+          config.home.confighome
+              or "${config.home.homeDirectory}/.config";
+        in [
+          # This directory should really be written in terms of
+          # `config.programs.nixcord.vesktopConfigDir`, but it defaults to null
+          # for unknown reasons.  This is in conflict with the docs, which claim
+          # it should be the path used here.
+          (lib.removePrefix
+            config.home.homeDirectory
+            "${xdg-config-home}/vesktop/sessionData")
+        ];
+
+      programs.nixcord = {
+        enable = true;
+        discord = {
+          # Install Vesktop without Discord.
+          enable = false;
+          # openASAR.enable = true;
+          vencord.enable = false;
+          vencord.package = pkgs.vencord;
+        };
+        vesktop = {
+          enable = true;
+          # package =
+          #   if cfg.fixWayland
+          #   then vesktop-wayland-wrapper
+          #   else pkgs.vesktop;
+        };
+      };
+      });
+}