feat: OAuth user fixes + dashboard UI improvements (v3.2.0)

- Bug A: Fix OAuth consent redirect loop (return_url path-relative)
- Bug B: Accept OAuth JWT tokens on /u/{user_id}/{alias}/mcp
- Bug C: Add user OAuth client CRUD (list/create/delete) + new page
- D-1: Sidebar border via Tailwind class, version footer in sidebar
- D-2: Remove broken setInterval targeting #stats-container
- D-3: 404.html purple primary colors + system dark mode support
- D-4: Fix invisible buttons (bg-gray-200 + text-white in light mode)
- D-5: Pin Alpine.js to 3.14.8, remove duplicate CSRF meta tag
- D-6: Fix invisible lang toggle button in settings (dark mode)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

core/templates/dashboard/health/index.html

@@ -316,7 +316,7 @@
 
         <div class="flex gap-2">
             <a href="/dashboard/health{% if lang and lang != 'en' %}?lang={{ lang }}{% endif %}"
-               class="px-4 py-2 bg-gray-200 dark:bg-gray-700 hover:bg-gray-300 dark:hover:bg-gray-600 rounded-lg text-white text-sm transition-colors">
+               class="px-4 py-2 bg-gray-200 dark:bg-gray-700 hover:bg-gray-300 dark:hover:bg-gray-600 rounded-lg text-gray-800 dark:text-white text-sm transition-colors">
                 {{ t.refresh }}
             </a>
             <a href="/dashboard/health?refresh=true{% if lang and lang != 'en' %}&lang={{ lang }}{% endif %}"