(feat): add browser audio playback preflight

[kyeunga25]

Summary

• verify supported files with browser-local playback metadata after the bounded container check
• show bilingual approximate duration and fail closed before private mutations when playback metadata is unreadable
• keep the probe in a dedicated lazy chunk with explicit build budgets and cleanup coverage

Validation

• 501 unit tests
• 140 Worker tests
• 79 client/provider plus 20 Worker local-AI tests
• 55 Chromium end-to-end tests, including 390 px unreadable-audio coverage
• production build and Wrangler dry-run
• public configuration check
• production and full dependency audits

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	studymix-ai	7f1274c	Aug 21 2026, 12:38 AM