Refactor DMS configuration and introduce home-manager integration

- Simplified `default.nix` by removing legacy home-manager settings and delegating them to a new `_private/hm.nix` module for better organization.
- Added a new script `dms-restart-once.sh` to handle service restarts and cache management for DMS plugins.
- Updated the DMS configuration to streamline the integration of third-party plugins and improve overall modularity.

modules/desktop/shells/dms/scripts/dms-restart-once.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+set -euo pipefail
+rt="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"
+exec 9>"$rt/dms-plugin-restart.lock"
+@FLOCK@ -n 9 || exit 0
+cache="${XDG_CACHE_HOME:-$HOME/.cache}/quickshell/qmlcache"
+rm -rf "$cache"
+@SYSTEMCTL@ --user try-restart dms.service 2>/dev/null || true