perf: clean up idle watcher listeners/interval to avoid duplicate background work

[Rohit-rk07]

Summary

_initIdleWatcher in js/activity.js adds multiple global event listeners and a periodic setInterval, but the cleanup path is not managed through Activity lifecycle teardown.

Code references

• js/activity.js:2952 (this._initIdleWatcher = () => { ... })
• js/activity.js:2972 window.addEventListener(mousemove, resetIdleTimer);
• js/activity.js:2973 window.addEventListener(mousedown, resetIdleTimer);
• js/activity.js:2974 window.addEventListener(keydown, resetIdleTimer);
• js/activity.js:2975 window.addEventListener(touchstart, resetIdleTimer);
• js/activity.js:2976 window.addEventListener(wheel, resetIdleTimer);
• js/activity.js:2979 setInterval(() => { ... }, 1000);

Why this matters

If Activity is reinitialized/reset in the same page session, additional listeners and intervals can accumulate, causing duplicated work, extra wakeups, and unnecessary CPU/battery use.

Expected behavior

• Track idle watcher listener handlers and interval id.
• Register listeners via managed lifecycle utilities (addEventListener tracker or equivalent).
• Clear the interval and detach listeners during teardown/reset.

Suggested fix

Introduce startIdleWatcher() / stopIdleWatcher() (or equivalent) and call stop in existing cleanup paths before reinit.

[yogibytes]

Hi @Rohit-rk07 👋

I've implemented a fix for issue #6140 that addresses the listener/interval accumulation problem.

What changed:

• Refactored _initIdleWatcher() to use managed this.addEventListener() tracking
• Stored listener handler and interval ID as instance properties for cleanup
• Added _stopIdleWatcher() method called during Activity re-initialization

PR: https://github.com/yogibytes/SugarLabs-musicblocks/pull/new/perf/fix-idle-watcher-cleanup

The fix leverages the existing Activity lifecycle cleanup mechanism and has zero breaking changes. Idle throttling functionality remains intact while eliminating duplicate background work.

Would appreciate your review and feedback!