chore: lazy load historical changelog videos

[Jocky-Teable]

Changes

• Keep the latest May 6 changelog videos loading normally.
• Lazy-load April and earlier 2026 changelog videos by moving their URLs to data-src until they are near the viewport.
• Reuse the existing changelog script with IntersectionObserver and a no-IntersectionObserver fallback.

Verification

• Ran git diff --check.
• Local rendered comparison: video=29, src=2, data_src=27, preload_auto=0, preload_metadata=2, preload_none=27.
• Online before comparison: video=29, src=29, data_src=0, preload_auto=2, preload_metadata=27, preload_none=0.

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