Screen sharing enabled by default in pre-join group lobby

### Steps to reproduce

1. Open a room containing an Element group call.
2. Click “Join”.
3. Observe that screen sharing is already enabled in the pre-join lobby.
4. Click “Join call” without actively disabling "Stop video".

### Outcome

**Actual result:**
The user joins the call while sharing their screen.
It also seems like the screenshare is being mirrored (?)

**Expected result:**
Screen sharing must default to disabled and should only be enabled by the user actively enabling it.

**Workaround:**
After clicking “Join”, manually disable screen sharing before clicking “Join call”.

### Operating system

Windows 11

### Application version

Element version: 1.12.24

### How did you install the app?

I do not remember. Maybe Winget?

### Homeserver

matrix.org

### Will you send logs?

Yes

Start at the pre-join group lobby and trace how screen-sharing state is initialized and passed into the “Join call” flow. Reproduce the issue using the listed steps, then verify that screen sharing is disabled by default and that joining without user action does not share or mirror the screen.

Written by the indexing model from the issue text.