fix(studio): single-frame export uses wrong content, ignores duration setting

[walliai666]

fix(studio): single-frame export uses wrong content, ignores duration setting

1. Wrong content: exportMp4 always rendered the template's original
   source HTML, ignoring the agent-generated preview saved to
   lastPreviewHtmlPath. Now uses lastPreviewHtmlPath when present.

2. Duration ignored: exportMp4 hardcoded duration: 'auto', causing
   the adapter to fall back to the animation's natural length (~4s)
   instead of the user's setting. durationTargetSec was defined in
   UserPreferences but never written or read.

   • studio-server now persists collected.duration to
     preferences.durationTargetSec at generate time (covers both
     single-frame and multi-frame paths).
   • exportMp4 reads durationTargetSec and passes it as
     durationMode: 'explicit' so the adapter honors it as a hard cap.

3. UI showed render time not video duration: the "MP4 已导出 · Xs"
   badge was displaying elapsed_ms (wall-clock render time) instead
   of the actual video length. export_done SSE event now carries
   duration_sec; the studio UI uses that instead.

Resolves #50
Resolves #51

[lefarcen]

Hey @walliai-chenlb! 👋

Nice work tracking down three distinct bugs in the same export path — the lastPreviewHtmlPath oversight, the hardcoded 'auto' duration, and the misleading badge all make sense together once you see the flow. The PR description is clear and well-scoped.

I've assigned @Siri-Ray for code review. In the meantime, this PR is linked to #50 which reports the same duration issue — good alignment.

[walliai666]

@lefarcen Thanks for the feedback and for linking this PR to #50!

I noticed there isn't currently a reviewer assigned on the PR, so just wanted to check whether the assignment to @Siri-Ray went through.

Also, just a small note: this PR addresses both #50 and #51.

I'll wait for the review and address any feedback once it's available. Thanks!

[lefarcen]

Hey @walliai-chenlb — yes, the review request to @Siri-Ray is live on the PR. Thanks for flagging that #51 is in scope too; that’s helpful context here.

@Siri-Ray could you take a look when you have a chance?