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

docs/dashboard-guide.md

@@ -92,6 +92,7 @@ Chat view provides project-scoped conversations with agents.
 - If a regular Chat stream drops with a hidden-tab/browser-suspension error (for example `Load failed`) while the server is still generating, Chat suppresses the false error banner, re-attaches to the in-progress stream using the durable replay state, and reconciles the final assistant reply when generation completes.
 - Chat message lists now track near-bottom scroll state: while you are reading older messages, live streaming/new replies do not force-scroll; a **Latest** jump control appears until you return to the tail.
 - On mobile direct-chat threads, entering a thread and restoring Chat after tab/page visibility returns re-anchors to the newest message (`scrollTop = scrollHeight`) so the view always opens at the live tail.
+- On mobile direct-chat threads, tapping the active title/identity in the thread header opens a lightweight conversation dropdown so you can switch to another direct session without backing out to the sidebar list first.
 - On mobile (`max-width: 768px`), chat bubbles are slightly wider in full Chat for improved readability while preserving header/composer gutters.
 - Full Chat tool-call summaries now use a denser mobile layout: grouped and single-call collapsed rows keep icon + label + status on one line (Quick Chat-style scanability) while expanded details remain unchanged.
 - The desktop Chat view toggle and mobile Chat tab now show an unread-response indicator when a live assistant reply arrives for your active chat thread after you leave Chat; opening Chat clears it immediately.