Fix download-gated extras visibility

[cursor]

Summary

• Show Stems & Downloads for download-only gated extras even when the full track is not public-downloadable
• Keep stem-only extras from rendering a full-track row while using fetched stems for rows/counts
• Prime parent _stems from useStems and fetch stems when premium purchase UI opens

Verification

• git diff --check HEAD~1..HEAD
• Unable to run npm/node-based checks because npm, npx, and node are not available on PATH in this environment

  

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Latest commit: 5b3f90b

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