Fix admin navigation URL endpoint error

PROBLEM DIAGNOSED:
- Admin panel was throwing BuildError: 'Could not build url for endpoint main.admin_settings'
- Template was trying to link to 'main.admin_settings' but actual route function is 'admin_system_settings'

ROOT CAUSE ANALYSIS:
- In app/main/routes.py line 789: route function is named 'admin_system_settings'
- In app/templates/main/admin.html line 70: template was calling 'main.admin_settings'
- URL endpoint mismatch caused Flask routing error

SOLUTION IMPLEMENTED:
- Updated admin.html template line 70 to use correct endpoint name
- Changed from: url_for('main.admin_settings')
- Changed to: url_for('main.admin_system_settings')

VERIFICATION:
- Route function 'admin_system_settings' exists and imports successfully
- URL endpoint now matches the actual route function name
- Admin panel should load without BuildError
- System Settings link should work correctly

This resolves the reported admin panel navigation error.

app/templates/main/admin.html

@@ -67,7 +67,7 @@
         <div class="admin-links">
             <a href="{{ url_for('main.admin_users') }}" class="btn">Manage Users</a>
             <a href="{{ url_for('main.admin_logs') }}" class="btn">View System Logs</a>
-            <a href="{{ url_for('main.admin_settings') }}" class="btn">System Settings</a>
+            <a href="{{ url_for('main.admin_system_settings') }}" class="btn">System Settings</a>
         </div>
 
         {% if system_stats %}