channelFull was computed only in the prescreen; a second click on the
channel in the room nav joined a full channel. Extract
useVoiceChannelFull, use it in both places, and refuse with a
"Channel full (N/N)" toast.
Fixes#30
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA