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/draft-preview/assets/preview.js

@@ -1,26 +0,0 @@
-(function(win, doc) {
-    'use strict';
-
-    if ( location.pathname.includes( 'pages' ) && document.querySelector( '#titlebar-save' ) )
-    {
-        // define preview URL and neighbour
-        let route = ( typeof draft_preview_route == 'string' ) ? draft_preview_route : '/preview';
-        let lang = ( typeof draft_preview_language == 'string' ) ? draft_preview_language : '';
-        const previewPath = lang + route + '?slug=/' + GravAdmin.config.route;
-        const ancestor = document.querySelector( '#titlebar-button-delete' );
-
-        // create button (clone of the original preview btn)
-        let button = doc.createElement( 'a' );
-        button.href = previewPath;
-        button.id = 'titlebar-button-preview';
-        button.target = "_blank";
-        button.classList.add( 'button' );
-        button.innerHTML = '<i class="fa fa-eye"></i>';
-
-        // insert button
-        ancestor.insertAdjacentElement( 'beforebegin', button );
-        // no CSS flex, so we need a space for spacing…
-        doc.querySelector( '.button-bar' ).insertBefore( doc.createTextNode( ' ' ), ancestor );
-    }
-
-})(window, document);