Export chat isn't exporting the whole visible timeline when you ask it to export the timeline

### Steps to reproduce

1. Open a DM with someone with a bunch of scrollback
2. Scrollback ~10 pages to the point you want to export from
3. Hit export chat for "the timeline"

### Outcome

I expect the 'visible' timeline which i can scroll through without additional backwards/forwards pagination to be exported.

In practice, only the most recent ~20 messages get exported.

It looks like export is not correctly spanning multiple timeline chunks, even if they're contiguous.

Without ability to export based on time range, i'm now a bit screwed.

### Operating system

macOS 12.1 on M1

### Application version

Nightly

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No