Element X Call: "Join call" Button Obstructs Camera Preview

### Your use case

### What would you like to do?
**1.** Have a clear preview of my video feed: "Join call" Button Obstructs Camera Preview.
**2.** Standardise lobby header: Room name is duplicated in header e.g. 'test' x2.
**3.** Make call participants visible in lobby.
**4.** Put camera preview in a container to match desktop design.
**5.** Center the letter of the default avatar.
**6.** Replace minimise button with a back button.

### How would you like to achieve it?
**1.** Have a clear preview of my video feed:
- Shrink preview window.
- Prevent the gradient effect from extending into camera preview.

**2.** Standardise lobby header: "Call Preview".
**3.** Make call participants visible in lobby: The 'members' icon was used.
**4.** Put camera preview in a container to match desktop design.
**5.** Center the letter of the default avatar.
**6.** Replace the minimise button with a back button.
### Image showing changes 1b, 2, 3 & 6

### Have you considered any alternatives?
**1.** Switch the position of the "Join call" button and media controls for a more streamlined visual scan.
- This alternative is due to the fact that media preferences aren't remembered.
- Adding an option to save preferences or last state would support the current layout better.

**2.** Standardise lobby header: _Room name_.
- Show user profile and display name as secondary.

### Additional context

Related to https://github.com/element-hq/element-call/issues/3254#issue-3048633583.

### Are you able to provide a PR?

No