feat(FN-3604): add chat unread indicator to header and mobile nav

Implements a chat unread indicator across the header and mobile nav bar, driven by a new unread-response tracker in the app state, with tests and a docs update for the feature.

Fusion-Task-Id: FN-3604

packages/dashboard/app/components/Header.css

@@ -624,6 +624,7 @@
   justify-content: center;
   width: 28px;
   height: 28px;
+  position: relative;
   background: none;
   border: none;
   border-radius: var(--radius-sm);
@@ -652,6 +653,12 @@
   color: var(--bg);
 }
 
+.header-chat-unread-dot {
+  position: absolute;
+  top: calc(var(--space-xs) * -1);
+  right: calc(var(--space-xs) * -1);
+}
+
 /* === View Toggle Overflow Dropdown === */
 .view-toggle-overflow-menu {
   position: absolute;