Screen share prompt scales poorly to short windows

### Steps to reproduce

1. Open Element Desktop, size window fairly short (~730px)
2. Start a 1:1 call or an embedded Element Call
3. Start a screen share

### Outcome

#### What did you expect?
Scroll area with the window to be shorter to fit the OK & cancel buttons in

#### What happened instead?
Scroll area within a scroll area, with the OK and cancel buttons scrolled off the bottom.

### Operating system

_No response_

### Application version

_No response_

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No

Start by reproducing the screen share prompt in Element Desktop at a window height of about 730px during a 1:1 call or embedded Element Call. Inspect the prompt's scroll areas and verify that the OK and Cancel buttons remain visible without a nested scroll area when the fix is complete.

Written by the indexing model from the issue text.