fix: keep heatmap overlay composited above the iframe snapshot in Safari

[CloudCorpRecords]

Fixes #4624.

In Safari, the heatmap click dots disappeared while scrolling because the iframe snapshot painted over the overlay.

The overlay is now promoted to its own compositing layer (translateZ(0) plus will-change), .snapshotClip gets isolation: isolate with z-index 0, and the scroll view's SnapshotPreview is wrapped in .snapshotClip for parity with the main view.

Note: as the reporter found, this does not reproduce in Playwright WebKit (it needs real Safari GPU compositing), so this fix is by code inspection of the compositing and stacking behavior.

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[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable regression was found.

Summary

Keeps the heatmap overlay above the iframe snapshot while scrolling in Safari.

• Heatmap marks stay above page snapshots while scrolling in Safari.

No actionable issues found. CloudCorpRecords states that the original bug requires real Safari GPU rendering and does not reproduce in Playwright WebKit. Real Safari behavior was not verified during this review.

_{Reviews (1) · Last reviewed commit: "fix: keep heatmap overlay composited abo..." · Reviewed by Greptile}