feat(models): route image model preparation through workers

[georgi]

Summary

• add the worker-advertised models.prepare path to the TypeScript bridge
• relay image-owned model preparation through the existing download WebSocket
• forward progress telemetry and backend selection through ModelDownloadStore

Validation

• focused ESLint (0 errors)
• Vitest: 52 passed
• ModelDownloadStore Jest: 14 passed
• runtime/websocket builds and typechecks passed locally