Add pitch preservation toggle to playback speed

[adaliea]

Summary

• add a remembered Preserve pitch checkbox to the playback-speed popover
• apply the preference immediately to the active HTML audio element
• keep pitch preservation enabled by default
• cover persistence, reset behavior, and audio-engine propagation

Validation

• pnpm typecheck
• pnpm exec biome check on changed files
• 58 focused unit tests
• 24 focused browser tests

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit 2bd5381.}

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	composer	2bd5381	Commit Preview URL Branch Preview URL	Sep 02 2026, 08:14 PM