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/_modals.scss

@@ -1,87 +0,0 @@
-// Modals
-.modal {
-  align-items: center;
-  bottom: 0;
-  display: none;
-  justify-content: center;
-  left: 0;
-  opacity: 0;
-  overflow: hidden;
-  padding: $layout-spacing;
-  position: fixed;
-  right: 0;
-  top: 0;
-
-  &:target,
-  &.active {
-    display: flex;
-    opacity: 1;
-    z-index: $zindex-4;
-
-    .modal-overlay {
-      background: rgba($bg-color, .75);
-      bottom: 0;
-      cursor: default;
-      display: block;
-      left: 0;
-      position: absolute;
-      right: 0;
-      top: 0;
-    }
-
-    .modal-container {
-      animation: slide-down .2s ease 1;
-      z-index: $zindex-0;
-    }
-  }
-
-  &.modal-sm {
-    .modal-container {
-      max-width: $control-width-sm;
-      padding: 0 $unit-2;
-    }
-  }
-
-  &.modal-lg {
-    .modal-overlay {
-      background: $bg-color-light;
-    }
-
-    .modal-container {
-      box-shadow: none;
-      max-width: $control-width-lg;
-    }
-  }
-}
-
-.modal-container {
-  @include shadow-variant(.2rem);
-  background: $bg-color-light;
-  border-radius: $border-radius;
-  display: flex;
-  flex-direction: column;
-  max-height: 75vh;
-  max-width: $control-width-md;
-  padding: 0 $unit-4;
-  width: 100%;
-
-  &.modal-fullheight {
-    max-height: 100vh;
-  }
-
-  .modal-header {
-    color: $dark-color;
-    padding: $unit-4;
-  }
-
-  .modal-body {
-    overflow-y: auto;
-    padding: $unit-4;
-    position: relative;
-  }
-
-  .modal-footer {
-    padding: $unit-4;
-    text-align: right;
-  }
-}