Implement Multi-threaded Agentic Duo Chat for Web UI

Currently, Agentic Duo Chat in the IDE and classic Duo Chat on the Web supports multi-threading functionality but this feature needs to be fully implemented and exposed in the web UI to match the functionality available in other platforms.

The codebase already contains the foundational components for multi-threaded chat:

• DuoChatThreads component for displaying thread history
• Thread management props (threadList, activeThreadId, multiThreadedView)
• Thread-related events (thread-selected, delete-thread, new-chat, back-to-list)