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/users/crumb/waybar.nix

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, ... }:
+
+let cfg = config.sydnix.users.crumb.waybar;
+in {
+  options.sydnix.users.crumb.waybar = {
+    enable = lib.mkEnableOption "Waybar, à la crumb";
+  };
+
+  config = lib.mkIf cfg.enable {
+    home.packages = [
+      pkgs.martian-mono
+      pkgs.nerd-fonts.symbols-only
+    ];
+
+    stylix.targets.waybar.enable = false;
+    programs.waybar = {
+      enable = true;
+      style = ./waybar/src/waybar.css;
+      systemd.enable = true;
+      systemd.target = "niri.service";
+      settings.sydbar = import ./waybar/src/waybar.nix {
+        niri-gaps = config.programs.niri.settings.layout.gaps;
+        inherit pkgs lib;
+      };
+    };
+  };
+}