Add helper functions for multi-user toggle support

Introduced a new file `lib.nix` containing helper functions to streamline user package management and attributes for multi-user configurations. Updated various modules to utilize these functions, enhancing code maintainability and readability.

parts/core.nix

@@ -213,6 +213,22 @@ in
         windows_vm = ../secrets/ssh/ed25519_windows_vm.pub;
       };
       getSshKeys = keyNames: keyNames |> map (name: inputs.self.lib.sshKeys.${name});
+      normalizeUsers = users:
+        if builtins.isString users then [ users ] else users;
+      mkUserPackages = lib: users: packages:
+        lib.mkMerge (map (user: {
+          ${user}.packages = packages;
+        }) (inputs.self.lib.normalizeUsers users));
+      mkUserAttrs = lib: users: attrs:
+        lib.mkMerge (map (user: {
+          ${user} = attrs;
+        }) (inputs.self.lib.normalizeUsers users));
+      mkHomeManagerUsers = lib: users: fn:
+        lib.mkMerge (map (user: {
+          ${user} = fn user;
+        }) (inputs.self.lib.normalizeUsers users));
+      getFirstUser = users:
+        if builtins.isString users then users else (builtins.head users);
     };
   };
 }