Implement the planner-multiplayer-awareness OpenSpec change:
- Add setUserName to use-yjs.ts that persists to localStorage and syncs via awareness
- Create ParticipantList component showing all session participants with color dots,
host badge, and inline name editing for the local user
- Replace plain text cursor labels with SVG pointer arrows and styled name tags
(colored background, drop shadow, rounded corners, z-index below map controls)
- Track awareness add/remove events and display auto-dismissing toasts (3s)
when participants join or leave the session
- Add i18n keys for participant UI in English and German
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>