(feat): add private local audio preview

[kyeunga25]

Summary

• add a native device-local preview only after both audio preflight gates pass
• pause, detach, and revoke temporary sources on replacement, clear, or unmount
• keep the validated file ready when optional preview creation is unavailable
• document the privacy boundary and add bilingual responsive lifecycle coverage

Verification

• format, strict typecheck, and lint
• 501 unit tests and 140 Worker tests
• 79 plus 20 local AI tests
• 57 Chromium end-to-end tests, including 390x844 layout
• production build and Worker dry run
• production and full dependency audits
• public configuration gate and count-only publication safety review

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	studymix-ai	cf87b0a	Aug 21 2026, 04:36 AM