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/problems/scss/_dropdowns.scss

@@ -1,36 +0,0 @@
-// Dropdown
-.dropdown {
-  display: inline-block;
-  position: relative;
-
-  .menu {
-    animation: slide-down .15s ease 1;
-    display: none;
-    left: 0;
-    max-height: 50vh;
-    overflow-y: auto;
-    position: absolute;
-    top: 100%;
-  }
-
-  &.dropdown-right {
-    .menu {
-      left: auto;
-      right: 0;
-    }
-  }
-
-  &.active .menu,
-  .dropdown-toggle:focus + .menu,
-  .menu:hover {
-    display: block;
-  }
-
-  // Fix dropdown-toggle border radius in button groups
-  .btn-group {
-    .dropdown-toggle:nth-last-child(2) {
-      border-bottom-right-radius: $border-radius;
-      border-top-right-radius: $border-radius;
-    }
-  }
-}