(fix): stabilize audio preview keyboard flow

[kyeunga25]

Summary

• give the native local preview an explicit keyboard focus stop
• wait for local preflight readiness before starting the keyboard path
• traverse bounded platform-specific media controls before continuing to preset and submission controls
• refresh the verified private bundle size

Verification

• format, strict typecheck, and lint
• 501 unit tests
• keyboard submission flow repeated 5 times in Chromium
• production web build and bundle budgets
• count-only publication safety review

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	studymix-ai	7d24057	Aug 21 2026, 04:52 AM