chore: Replace non-null assertions with proper error handling in RealAppsEngineUIHost

Description

The file client/apps/RealAppsEngineUIHost.ts currently contains multiple non-null assertions (!) that are marked with a // FIXME comment. These assertions are unsafe and bypass strict null checks, which can lead to potential runtime errors if the expected data is missing.

Current Behavior

The code uses ! to force non-null types for:

• getUserAvatarURL(username)
• room.name / slugifiedName
• getUser() result
• Member username in getClientRoomInfo

Expected Behavior

The non-null assertions should be replaced with proper null checks and error handling. If a required value is missing (e.g., user not logged in, room name undefined), the code should handle it gracefully or throw a meaningful error instead of risking a crash.

Tasks

• Locate RealAppsEngineUIHost.ts in client/apps/.
• Remove all ! non-null assertions.
• Implement explicit checks for undefined or null values.
• Remove the // FIXME comment.

Start by reading client/apps/RealAppsEngineUIHost.ts and the referenced getUserAvatarURL, getUser, room, and getClientRoomInfo call sites. Identify how missing users, room names, avatar URLs, and member usernames are represented before choosing explicit handling. Done means all listed non-null assertions and the FIXME comment are removed and missing values are handled explicitly.