[FR] AI chat mention a page UI improvments

### Description

Even though we've disabled the mention a page feature recently, we plan to re-enable it in the future. Below are some aspects UI that could be improved when we get there:

- If there is a mention page in the input, ctrl + left or right arrow key to jump to the previous or next word doesn't behave correctly.
- Sometimes, the mention page menu after inserting "@" shows "no page results" when there are actually documents
- loading indicator for sending a long message could use some visual upgrade
- add some space between consecutive mentioned pages in the ai prompt input
- styling for mentioned pages in user-sent messages.

### Impact

AI Chat users

### Additional Context

_No response_