Assistant UI UX tweaks

[Regaddi]

This PR updates the AI Assistant UI and reduces the font-size for closed captions from 20px to 14px.

Here's a screenshot of the updated UI:

Changes outlined:

• Summaries are added to the chat stream and are highlighted with an orange-y color
• We don't add the same summary text twice in a row, meaning that spamming the "Get summary" button, won't add duplicate messages to the chat stream.
• When the same summary is already available in the chat stream, its date is updated and the summary is being moved to the bottom of the chat stream.

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Name	Status	Preview	Updated (UTC)
daily-ai-assistant-demo	✅ Ready (Inspect)	Visit Preview	Jan 11, 2024 1:57pm

[Regaddi]

Although we decided to simplify the assistant tab -- it now feels very "empty" or airy before the first summary is generated.

@chobberoni this might be less of an issue combined with the automatic summary request when joining a meeting, which is part of Liza's PR at #21.

We might also show a placeholder card when there are no messages, something like "It's pretty quiet here, right? Why don't you start the conversation?"