fix: align downloader progress UI

[Priveetee]

Summary

• support the Go downloader status/stage contract with terminal state driven by status
• replace the old downloader waiting/metrics panel with a cleaner user-facing progress card
• keep artifact opening on the canonical /api/downloader/jobs/{id}/artifact route

Validation

• bun run check
• bun run knip
• bun run sherif
• bun run build