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/_toasts.scss

@@ -1,42 +0,0 @@
-// Toasts
-.toast {
-  @include toast-variant($dark-color);
-  border: $border-width solid $dark-color;
-  border-radius: $border-radius;
-  color: $light-color;
-  display: block;
-  padding: $layout-spacing;
-  width: 100%;
-
-  &.toast-primary {
-    @include toast-variant($primary-color);
-  }
-
-  &.toast-success {
-    @include toast-variant($success-color);
-  }
-
-  &.toast-warning {
-    @include toast-variant($warning-color);
-  }
-
-  &.toast-error {
-    @include toast-variant($error-color);
-  }
-
-  a {
-    color: $light-color;
-    text-decoration: underline;
-    
-    &:focus,
-    &:hover,
-    &:active,
-    &.active {
-      opacity: .75;
-    }
-  }
-
-  .btn-clear {
-    margin: 4px -2px 4px 4px;
-  }
-}