feat: add remote click indicator pulse on shape stream start

- Show a subtle green circle near remote cursor when a user initiates drawing
- Triggered via STREAM_SHAPE WebSocket message
- Automatically fades out after 800ms
- Added `remoteClickIndicators` map to track timestamp of last shape stream per user-connection
- Cleaned up expired indicators in clearCanvas
- Refactored remote cursor rendering to use userConnKey from Map key

Author: coderomm