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.

users/crumb/files.nix

@@ -33,6 +33,7 @@ in lib.mkMerge [
         "Videos"
         "src" # My projects.
         "scratch" # My playgrounds.
+        "git" # Other peoples' projects.
         # Should "org" be declared in emacs.nix?  I don't think so.  I
         # conjecture that my org files are extremely valuable with or without
         # Emacs.
@@ -40,27 +41,15 @@ in lib.mkMerge [
         # REVIEW: I think it may be preferable to persist a few individual files
         # under ~/.ssh, rather than the whole directory.
         ".ssh"
-        {
-          directory = ".local/share/Steam";
-          method = "symlink";
-        }
-        ".passage"
+        # FIXME: This is only temporary.  KDE Plasma can be a PITA,
+        # unpredictably scattering configuration files all about the file
+        # system.  Until we switch to Niri, we'll just persist the whole
+        # directory.  WAIT4NIRI
+        # ".config"
       ];
     };
   }
 
-  {
-    # ~/git — other peoples' code
-    systemd.user.tmpfiles.rules = [
-      "v ${config.home.homeDirectory}/git 755 ${config.home.username} users - -"
-    ];
-
-    sydnix.impermanence.directories = [
-      "git/doomemacs"
-      "git/publish-org-roam-ui"
-    ];
-  }
-
   {
     ### Syncthing
 
@@ -79,6 +68,13 @@ in lib.mkMerge [
           ];
           ignorePerms = true;
         };
+        "Music" = {
+          path = "~/Music";
+          devices = [
+            "deertopia"
+          ];
+          ignorePerms = true;
+        };
       };
     };
   }