Fix online users page always showing /InAppNotifications/Recent as current page

The notification bell polls /InAppNotifications/Recent (a JSON endpoint) every time
it loads. Because the middleware throttles updates to once per 60s, the update fired
on whichever request first arrived after the throttle expired — usually the bell poll
rather than a real page navigation. Fix: skip any response whose Content-Type is
application/json so only full page navigations update the current-page field.

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

src/PowderCoating.Web/Middleware/OnlineUserMiddleware.cs

@@ -50,6 +50,12 @@ public class OnlineUserMiddleware
     {
         await _next(context);
 
+        // Skip AJAX/JSON responses — they are not page navigations and would
+        // cause the "current page" to show the polling endpoint (e.g. /InAppNotifications/Recent)
+        // rather than the actual page the user is on.
+        if (context.Response.ContentType?.Contains("application/json", StringComparison.OrdinalIgnoreCase) == true)
+            return;
+
         // Only track authenticated, non-API, non-asset requests
         if (!context.User.Identity?.IsAuthenticated ?? true) return;
         var path = context.Request.Path.Value ?? string.Empty;