Fix: pause widget animations off screen and stop VPN loop stacking

[SandObserver]

Summary

• Add whileOnScreen(el, onChange) to the widget toolbox: reports whether an element intersects the viewport in a visible tab, through IntersectionObserver and visibilitychange.
• Gate the Connections map pulse loop and the VPN geo pulse loop on it. A refresh off screen still paints one frame.
• Store the VPN frame id on every request and clear it at the start of each paint, so a render cancels the running loop instead of starting another.
• Keep the Analog clock stopped on visibilitychange while its face is off screen.
• Refresh the cache-busting stamps.

Testing
All green. Added tests for whileOnScreen and for the map pausing off screen and resuming on return.

Changelog

Fixed

• Stop the Connections VPN view from stacking animation loops while its page is off screen.
• Pause the Analog clock, Connections map and VPN animations on dashboard pages not shown.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.