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/langswitcher/css/langswitcher.css

@@ -1,22 +0,0 @@
-.langswitcher {
-    position: relative;
-    top: 50%;
-    -webkit-transform: translateY(-50%);
-    -moz-transform: translateY(-50%);
-    -o-transform: translateY(-50%);
-    -ms-transform: translateY(-50%);
-    transform: translateY(-50%);
-    margin-left: 1rem !important;
-    display: inline-block;
-}
-
-.langswitcher li {
-    display: inline-block;
-    margin-left: 0.5rem;
-    line-height: 1rem;
-}
-
-.langswitcher .active {
-    font-weight: bold;
-    text-decoration: underline;
-}