Add Online Now shortcut to Platform Admin nav with live user count

Adds a direct link to UserActivity/Online immediately below the People
& Activity hub entry so the current active-user count is visible at a
glance without navigating into the hub first. The count badge is
rendered using the already-injected OnlineUserTracker.GetActiveCount()
call and is hidden when the count is zero.

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

src/PowderCoating.Web/Views/Shared/_Layout.cshtml

@@ -1294,6 +1294,15 @@
                         <i class="bi bi-people"></i>
                         <span>People &amp; Activity</span>
                     </a>
+                    <a asp-controller="UserActivity" asp-action="Online" class="nav-link">
+                        <i class="bi bi-broadcast-pin"></i>
+                        <span>Online Now</span>
+                        @{ var _onlineCount = OnlineUserTracker.GetActiveCount(); }
+                        @if (_onlineCount > 0)
+                        {
+                            <span class="badge bg-success ms-auto">@_onlineCount</span>
+                        }
+                    </a>
                     <a asp-controller="PlatformAdmin" asp-action="ContentMessaging" class="nav-link">
                         <i class="bi bi-megaphone"></i>
                         <span>Content &amp; Messaging</span>