Room avatar upload modal: circular preview and drag-and-drop shares file in room

[mremond]

Description

Two issues have been identified with the room avatar upload modal ("Change avatar" dialog).

1. Avatar preview shape

When changing a room avatar, the preview in the modal is displayed as a circle. It should instead be displayed as a rounded square, matching the shape used for room avatars elsewhere in the application.

• Expected: The avatar preview is shown as a rounded square, consistent with the room avatar style.
• Actual: The avatar preview is shown as a circle.

2. Drag and drop misdirected

Dragging and dropping a file onto the avatar upload modal uploads the file to be shared in the room, rather than setting it as the avatar.

• Expected: Dropping a file onto the modal sets it as the avatar preview (or is otherwise handled as an avatar source).
• Actual: The dropped file is uploaded and shared in the room.

Additional Context

The avatar preview for users should stay a circle.