Switch default transport to WebSocket

- Dockerfile builds with --features websocket and sets VITE_TRANSPORT=websocket
- Server default feature changed from socketio to websocket
- Binary canvas transfer (1MB) instead of Socket.IO JSON (~3-4MB per connection)
- Significantly reduces memory pressure under concurrent connections

crates/server/Cargo.toml

@@ -4,7 +4,7 @@ version.workspace = true
 edition.workspace = true
 
 [features]
-default = ["socketio"]
+default = ["websocket"]
 socketio = ["dep:socketio", "dep:socketioxide"]
 websocket = ["dep:websocket"]