Admin improvements, RBAC and styling fixes

Extend admin interface:

• Make admin controllers inherit from Admin::BaseController
• New admin dashboard with system statistics overview
• Centralized admin layout with dedicated navigation
• Enhanced user management interface with role assignment capabilities

RBAC fixes:

• Introduced new admin role that has all rights
• Added db migration to create admin role with appropriate permissions
• Updated admin rake tasks for role management
• Updated db seeds to add all required roles properly

Styling improvements and fixes:

• Upgraded button system with standardized styling and dark mode support
• Improved UI with consistent styling using Tailwind components
• Brought back manual dark mode using Tailwind custom variant

app/assets/stylesheets/application.tailwind.css

@@ -2,20 +2,59 @@
 @tailwind components;
 @tailwind utilities;
 
-/*
+@custom-variant dark (&:where(.dark, .dark *));
 
 @layer components {
+  /* Button Base Styles */
+  .btn {
+    @apply py-2 px-4 font-medium rounded inline-block text-center no-underline border border-transparent cursor-pointer;
+  }
+
+  /* Primary Button */
   .btn-primary {
-    @apply py-2 px-4 bg-blue-200;
+    @apply bg-blue-600 text-white hover:bg-blue-700 dark:bg-blue-700 dark:hover:bg-blue-800;
+  }
+
+  /* Secondary Button */
+  .btn-secondary {
+    @apply bg-gray-200 text-gray-800 border-gray-300 hover:bg-gray-300 dark:bg-gray-600 dark:text-gray-200 dark:border-gray-700 dark:hover:bg-gray-700;
+  }
+
+  /* Tertiary Button */
+  .btn-tertiary {
+    @apply bg-gray-600 text-white hover:bg-gray-700 dark:bg-gray-700 dark:hover:bg-gray-800;
+  }
+
+  /* Danger Button */
+  .btn-danger {
+    @apply bg-red-600 text-white hover:bg-red-700 dark:bg-red-700 dark:hover:bg-red-800;
+  }
+
+  /* Success Button */
+  .btn-success {
+    @apply bg-green-600 text-white hover:bg-green-700 dark:bg-green-700 dark:hover:bg-green-800;
+  }
+
+  /* Info Button */
+  .btn-info {
+    @apply bg-indigo-600 text-white hover:bg-indigo-700 dark:bg-indigo-700 dark:hover:bg-indigo-800;
+  }
+
+  /* Light Info Button */
+  .btn-info-light {
+    @apply bg-indigo-100 text-indigo-700 hover:bg-indigo-200 dark:bg-indigo-900 dark:text-indigo-200 dark:hover:bg-indigo-800;
   }
-}
 
-*/
+  /* Light Success Button */
+  .btn-success-light {
+    @apply bg-green-100 text-green-700 hover:bg-green-200 dark:bg-green-900 dark:text-green-200 dark:hover:bg-green-800;
+  }
+}
 
 h1, h2, h3, h4, h5, h6 {
   @apply font-bold;
 }
-input[type=submit] {
+{{/* input[type=submit] {
   @apply border bg-gray-400 rounded-md px-2 py-1;
   &.primary {
     @apply bg-teal-800 text-teal-50 border-teal-600;
@@ -37,7 +76,7 @@ select, [type=text], [type=password] {
       @apply bg-white/60;
     }
   }
-}
+} */}}
 
 .session-holder {
   @apply w-full;
@@ -79,11 +118,6 @@ select, [type=text], [type=password] {
     }
   }
 }
-.main-nav {
-  a {
-    @apply underline hover:text-blue-600 hover:border-blue-600;
-  }
-}
 .past {
   @apply opacity-50 hover:opacity-100;
 }