home: create 'modules/home' folder

Consolidating all modules under the same path, to clear out the
top-level directory.

modules/home/mail/himalaya/default.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.home.mail.himalaya;
+in
+{
+  config.programs.himalaya = lib.mkIf cfg.enable {
+    enable = true;
+
+    settings = {
+      notify-cmd =
+        let
+          notify-send = lib.getExe pkgs.libnotify;
+        in
+        pkgs.writeScript "mail-notifier" ''
+          SENDER="$1"
+          SUBJECT="$2"
+          ${notify-send} \
+            -c himalaya \
+            -- "$(printf 'Received email from %s\n\n%s' "$SENDER" "$SUBJECT")"
+        '';
+    };
+  };
+}