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.

flake.nix

@@ -12,6 +12,10 @@
     sydnix-cli.url = "path:///persist/dots/scripts/sydnix-cli";
     copyparty.url = "github:9001/copyparty";
     emacs-overlay.url = "github:nix-community/emacs-overlay";
+    niri.url = "github:sodiboo/niri-flake";
+    stylix.url = "github:danth/stylix";
+    # nixcord.url = "github:kaylorben/nixcord";
+    nixcord.url = "github:msyds/nixcord/irc-colours";
     # Used for Firefox extensions/addons.
     nur = {
       url = "github:nix-community/NUR";
@@ -61,9 +65,11 @@
   nixConfig = {
     extra-substituters = [
       "https://nix-community.cachix.org"
+      "https://niri.cachix.org" # WAIT4NIRI
     ];
     extra-trusted-public-keys = [
       "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      "niri.cachix.org-1:Wv0OmO7PsuocRKzfDoJ3mulSl7Z6oezYhGhR+3W2964=" # WAIT4NIRI
     ];
   };
 }