Fix removing currently playing song from catalog

[LinaBell]

Summary

• When the currently playing song is removed from the catalog, the canvas media now stops immediately and skips to the next available song in the queue
• If no songs remain in the queue, media is set to none and nowPlaying is cleared
• Publishes a nowPlaying SSE event so all connected clients update immediately
• Previously, removing a playing song only updated the data object without stopping the canvas media, causing the removed song to continue playing, loop, or create desync between UI and audio

Test plan

• Play a song, remove it from catalog while playing — verify it stops and next song starts
• Play a song with nothing in queue, remove from catalog — verify media stops completely
• Play a song, remove a different song from catalog — verify currently playing is unaffected
• As non-admin user, verify audio stops when admin removes the currently playing song
• Remove from queue (not catalog) — verify existing behavior unchanged

Generated with Claude Code (https://claude.com/claude-code)