fix: address review comments for analytics tracking

[Copilot]

Addressed two edge cases identified in code review for the scroll depth tracking implementation:

Division by zero protection

• Added check for docHeight <= 0 to handle pages where content fits within viewport
• Returns 100% scroll instead of NaN/Infinity

Memory leak cleanup

• Remove scroll event listener once all thresholds (25%, 50%, 75%, 100%) are tracked
• Prevents unnecessary handler invocations after tracking is complete

const scrollPercent =
  docHeight <= 0 ? 100 : Math.round((scrollTop / docHeight) * 100)

// Remove listener once all thresholds have been tracked
if (firedThresholds.size === scrollThresholds.length) {
  window.removeEventListener('scroll', handleScroll)
}

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[lmammino]

Already being addressed in #30