refactor: use tabs macro for settings page with mobile support

- Replace custom sidebar/icon nav with standard tabs_nav macro
- Add horizontal scroll for mobile (overflow-x-auto)
- Reduce tab spacing on mobile (space-x-4 vs space-x-8 on desktop)
- Content panels now take full width below tabs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/shared/macros/tabs.html

@@ -12,7 +12,7 @@
 {% macro tabs_nav(tab_var='activeTab', class='') %}
 <div class="mb-6 {{ class }}">
     <div class="border-b border-gray-200 dark:border-gray-700">
-        <nav class="-mb-px flex space-x-8">
+        <nav class="-mb-px flex space-x-4 md:space-x-8 overflow-x-auto">
             {{ caller() }}
         </nav>
     </div>