Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/admin/themes/grav/app/utils/offline.js

@@ -1,21 +0,0 @@
-import $ from 'jquery';
-import isOnline from '../utils/offline';
-
-const offlineElement = $('#offline-status');
-
-$(window).on('offline', () => {
-    offlineElement.slideDown();
-});
-
-$(window).on('online', () => {
-    offlineElement.slideUp();
-});
-
-$(document).ready(() => {
-    if (!isOnline) {
-        offlineElement.slideDown();
-    }
-});
-
-// assume online if can't check
-export default typeof global.navigator.onLine !== 'undefined' ? global.navigator.onLine : true;