Make chat_app refresh its prepopulated UI messages on reload

[jcheng5]

Before this commit:

1. Call chat_app(client) (or ellmer::live_browser(client), which passes through to chat_app)
2. Chat with the bot a little bit
3. Hit Reload in the browser window

The discussion you have disappears, but the state is still present in the client--the UI and server state have gotten out of sync.

If you Ctrl+C out of the app, and then call chat_app(client) again, the existing messages are now displayed. But if you talk more, and then hit Reload, you'll be reverted back to the UI state that was correct at the time chat_app(client) was most recently called.

This is fixed by calling chat_ui at page load time, not at chat_app() invocation time.

(Note that it's unsafe to use a chat_app() from multiple browser windows simultaneously, since every chat will mutate the same client object. Might be worth putting in a tiny bit of code that closes existing sessions when a new session arrives.)

[gadenbuie]

Great catch and simple fix. Thanks!