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/emacs/modules/syd-scratch.el

@@ -30,7 +30,8 @@
 (use-package emacs
   :custom ((initial-scratch-message
             (with-temp-buffer
-              (insert-file-contents "/persist/vault/crumb/cool-deer.org")
+              (insert-file-contents (file-name-concat user-emacs-directory
+						      "cool-deer.org"))
               (buffer-string)))
            ;; Initial major mode for scratch buffer.
            (initial-major-mode 'fundamental-mode)