Allow threads to run in the background

[AbhinRustagi]

Issue: Currently if an assistant is streaming a request in a thread, and the user switches the chat, the request is aborted and abandoned - bad UX.

Implemented solve:

1. Update the store to supports multiple ThreadState
2. Added isCreatingThread guard: When you send the first message in a brand-new chat, processMessage runs under DRAFT_KEY and then awaits createThread to get a real thread id. During that brief window, the draft slot is "owned" by that in-flight run. If you hit New Chat again during that window, switchToNewThread would clear/reset DRAFT_KEY.
3. Added a check to show a loader when a request moves to background as the user switches to another thread.
4. Implemented LRU Cache for thread memory management