refactor(ui-server): unify HTTP and WebSocket payload size limits

refactor(ui-server): unify HTTP and WebSocket payload size limits

Merge DEFAULT_MAX_BODY_SIZE and DEFAULT_WS_MAX_PAYLOAD into single
DEFAULT_MAX_PAYLOAD_SIZE constant (1MB). Both represent the same
semantic: UI protocol payload limit over different transport layers.

Also increases WebSocket limit from 100KB to 1MB to match HTTP,
allowing ~200-300 charging stations in list responses.