fix: missing early return in validateMultiTrackState, division by zero in frame-export, stale closure in useFontManager

[saurabhhhcodes]

Real fixes:

1. timeline.ts: validateMultiTrackState continues past type check - TypeError when timelineTracks is not array
2. frame-export.ts: captureFrameAsPng has no guard for 0-dimension video - divide by zero, NaN scale
3. useFontManager.ts: stale closure in addFonts allows duplicate font entries on concurrent calls

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⚠️ PR Format Issues — @saurabhhhcodes

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👋 Thanks for your PR, @saurabhhhcodes!

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