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/_navbar.scss

@@ -1,29 +0,0 @@
-// Navbar
-.navbar {
-  align-items: stretch;
-  display: flex;
-  flex-wrap: wrap;
-  justify-content: space-between;
-
-  .navbar-section {
-    align-items: center;
-    display: flex;
-    flex: 1 0 0;
-
-    &:not(:first-child):last-child {
-      justify-content: flex-end;
-    }
-  }
-
-  .navbar-center {
-    align-items: center;
-    display: flex;
-    flex: 0 0 auto;
-  }
-
-  .navbar-brand {
-    font-size: $font-size-lg;
-    font-weight: 500;
-    text-decoration: none;
-  }
-}