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/templates/preview.html.twig

@@ -1,24 +0,0 @@
-{% extends 'partials/base.html.twig' %}
-
-{% set route = admin.route == '' ? '/' : '/' ~ admin.route %}
-{% set preview_link = base_url_relative_frontend|rtrim('/') ~ route %}
-{% set preview_html = (base_url|rtrim('/') ~ '/preview' ~ (context.home ? '' : context.route)) ?: '/' %}
-{% set admin_route = '/' ~ admin.route %}
-{% set back_link = base_url|rtrim('/') ~ '/pages' ~ page.find(admin_route).rawRoute() %}
-
-{% block titlebar %}
-    <div class="button-bar">
-        <a class="button" href="{{ back_link }}"><i class="fa fa-reply"></i> {{ "PLUGIN_ADMIN.BACK"|t }}</a>
-        <a class="button" href="{{ preview_link }}" target="_blank"><i class="fa fa-external-link"></i> {{ "PLUGIN_ADMIN.OPEN_NEW_TAB"|t }}</a>
-    </div>
-    <h1><i class="fa fa-fw fa-eye"></i> {{ "PLUGIN_ADMIN.PREVIEW"|t }}: <strong>/{{ admin.route }}</strong></h1>
-{% endblock %}
-
-{% block content_wrapper %}
-
-    <div class="content-wrapper preview-wrapper">
-        <div class="content-padding" width="100%" height="100%">
-            <iframe width="100%" height="100%" frameborder="0" src="{{ preview_link }}" />
-        </div>
-    </div>
-{% endblock %}