Multi-chat: replace easily-overlooked tips banner with a better first-run introduction or empty state

Multi-chat currently relies on a tips banner to introduce the feature, but it is easy to overlook and doesn't give new users a clear sense of what multi-chat is or how to use it when opening the tab,

**Suggestions:**
- Show a one-time introduction/spotlight the first time a user opens multi-chat, rather than (or in addition to) a banner that can be missed or dismissed without being read.
- Alternatively, replace the banner with a proper empty state for the multi-chat view that matches the style of other empty states in the product (e.g. icon + heading + short description + primary action), rather than a thin tips strip.

**Expected behavior:** New users discover and understand the multi-chat feature through a more prominent, consistent first-run experience or empty state.

**Actual behavior:** The feature is introduced via a tips banner that is easy to miss.