fix: remove synchronous media metadata XHR from render-critical paths

[ubugeeei]

Finding

Media metadata helpers use synchronous XHR for video/audio durations. This can freeze the UI/headless renderer and cache failed zero durations.

Evidence

• src/lib/video/video.tsx metadata helper.
• src/lib/sound/sound.tsx metadata helper.
• src/lib/vue.tsx has analogous logic.

Acceptance criteria

• Replace sync XHR with async metadata loading or preflight planning.
• Avoid permanently caching failure values.
• Preserve deterministic render startup.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。