Time Spent Gooning (TSG) Leaderboard Metric

Add a leaderboard metric for time spent gooning to bobina: Implement it by combining viewport detection, page visibility, and an idle timer. Attach an IntersectionObserver to the image so you know when at least a chosen percentage (for example 70%) of it is inside the viewport. At the same time maintain a simple idle detector by listening for user activity events such as mousemove, pointerdown, keydown, touchstart, and scroll, updating a lastActivity timestamp whenever they occur. If the image is visible, the tab is still visible (document.visibilityState === "visible"), and the time since lastActivity exceeds your idle threshold (for example 15 seconds), start accumulating time. Stop accumulating if the image leaves the viewport, the tab becomes hidden, the window loses focus, or new activity occurs. Periodically send the accumulated idle-view time to your backend and aggregate it there to power the leaderboard.