accessibility: element call embedded widget not in a modal dialog

### Steps to reproduce

1. on element logged in, go to a direct message room, or a public room but a dm would be best
2. Press the video call button
3. Press on element call, not legacy call

### Outcome

#### What did you expect?

the iframe should instead be a modal dialog, either with a native html element or by using divs with the proper aria roles and navigation patterns. In particular:

* it must immediately gain focus
* pressing a close dialog button should minimise the call widget and allow us to return to chat
* the regular interface of the app beyond the modal should not be visible, to the screenreader I mean

#### What happened instead?

The embedded widget appears in a container, but which blends in with the UI. This may or may not look good from a visual point of view, but it makes the application harder to use

For screenreaders which support navigating by frame, this is relatively easy, go to the top of the window and press the shortcut to get there. Otherwise though, it's not a pleasent experience and makes the user interface seemn more cluttered than it already is. Furthermore, not all screenreaders support navigating between all containers, especially frames because those aren't very common, so many screenreader vendors don't think it necesary to implement a shortcut for that.

### Operating system

arch linux, but that shouldn't matter

### Browser information

firefox latest, updated yesterday

### URL for webapp

_No response_

### Application version

Element version: 1.11.100 Crypto version: Rust SDK 0.10.0 (3cc301d), Vodozemac 0.9.0

### Homeserver

synapse, version is not applicable

### Will you send logs?

No