UX: simplify running source volume copy warning - #59

Summary

• remove the "Allow copying while source workspace is running" checkbox
• always allow live source copying from the UI and keep the warning as the operator signal
• move the running-source warning directly below the Source block for symmetry with Destination
• keep transitional source states blocked
• remove the obsolete draft state for the checkbox

Validation

• pnpm run lint:types
• pnpm exec biome check ...
• 12/12 WorkspaceVolumeCopyPage unit tests
• git diff --check

No deployment changes.