feat(FN-4042): add mobile chat quick switcher to thread header

Added a mobile chat quick switcher accessible from the thread header, with styling for the session dropdown and test coverage. Documentation for the feature was added to MOBILE.md and the dashboard guide.

Fusion-Task-Id: FN-4042

packages/dashboard/app/components/ChatView.css

@@ -377,6 +377,101 @@
   min-width: 0;
 }
 
+.chat-mobile-session-menu {
+  position: relative;
+  min-width: 0;
+  max-width: 100%;
+}
+
+.chat-mobile-session-trigger {
+  width: 100%;
+  max-width: 100%;
+  display: inline-flex;
+  align-items: center;
+  justify-content: flex-start;
+  gap: var(--space-sm);
+  border: none;
+  border-radius: var(--radius-md);
+  background: transparent;
+  color: var(--text);
+  padding: var(--space-xs) var(--space-sm);
+  min-height: calc(var(--space-lg) * 2);
+}
+
+.chat-mobile-session-trigger .chat-thread-header-title,
+.chat-mobile-session-trigger .chat-model-tag {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.chat-mobile-session-trigger svg:last-child {
+  margin-left: auto;
+  color: var(--text-muted);
+  flex-shrink: 0;
+}
+
+.chat-mobile-session-trigger:hover {
+  background: var(--card-hover);
+}
+
+.chat-mobile-session-trigger:focus-visible {
+  outline: none;
+  box-shadow: var(--focus-ring-strong);
+}
+
+.chat-mobile-session-dropdown {
+  position: absolute;
+  top: calc(100% + var(--space-xs));
+  left: 0;
+  right: 0;
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-xs);
+  padding: var(--space-xs);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-md);
+  background: var(--surface);
+  box-shadow: var(--shadow-lg);
+  z-index: 4;
+  max-height: calc(var(--space-xl) * 10);
+  overflow-y: auto;
+}
+
+.chat-mobile-session-option {
+  width: 100%;
+  display: flex;
+  align-items: center;
+  min-height: calc(var(--space-lg) * 2);
+  border: none;
+  border-radius: var(--radius-sm);
+  background: transparent;
+  color: var(--text);
+  cursor: pointer;
+  text-align: left;
+  padding: var(--space-sm) var(--space-md);
+}
+
+.chat-mobile-session-option:hover {
+  background: var(--card-hover);
+}
+
+.chat-mobile-session-option:focus-visible {
+  outline: none;
+  box-shadow: var(--focus-ring-strong);
+}
+
+.chat-mobile-session-option--active {
+  background: color-mix(in srgb, var(--todo) 12%, transparent);
+}
+
+.chat-mobile-session-option-title {
+  width: 100%;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
 .chat-thread-header-new-chat {
   flex-shrink: 0;
 }
@@ -1343,7 +1438,11 @@
     flex: 1 1 auto;
     min-width: 0;
     white-space: nowrap;
-    overflow: hidden;
+    overflow: visible;
+  }
+
+  .chat-mobile-session-menu {
+    width: 100%;
   }
 
   .chat-thread-header-identity .chat-thread-header-title,