Export Chat feature: add more details about the size limit and include attachments UI elements

### Your use case

#### What would you like to do?

The export chat feature is great! However, the "Size Limit" does not explain what is being limited: Is the total export size limited to the specified number of megabytes?

There is an "include attachments" checkbox below, are attachments included in the size limit? Are inline images considered attachments?

Additionally, why is there a limit of 2000 MB on the export? While would be quite a large conversation, presumably somebody wanting to export a message would want everything.

#### Why would you like to do it?

Adding this information to the UI dialog will document the features so that the user knows what is being exported.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_