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/syncthing.nix

@@ -40,6 +40,7 @@ in {
   };
 
   config = lib.mkIf cfg.enable {
+
     # TODO: sydnix.syncthing module
     sydnix.impermanence.directories =
       let xdg-state-dir =