Fix download section labels without stems

[raymondjacobson]

Summary

• Show "Download"/"DOWNLOAD" instead of "Stems & Downloads" when a track has no stems
• Hide expand/caret affordances for single full-track download cases
• Keep "Stems & Downloads" copy and download-all behavior when stems are present

Verification

• git diff --check

Note: Package typecheck/format scripts were not run because npm/npx/pnpm/yarn/tsc are unavailable on PATH in this worktree.

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Latest commit: 3b01316

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