Fix scroll event listener memory leak in homepage header

[Copilot]

Each page transition was adding a new scroll listener without removing the previous one, causing memory leaks and multiple checkScroll executions per scroll event.

Changes

• Use AbortController to manage scroll listener lifecycle
• Abort previous controller before creating new one on page transitions
• Pass controller signal to addEventListener to auto-remove listeners when aborted

let scrollController: AbortController | null = null;

function initHomepageHeader() {
    if (scrollController) {
        scrollController.abort();
    }
    scrollController = new AbortController();
    
    // ... setup code ...
    
    window.addEventListener('scroll', onScroll, {
        passive: true,
        signal: scrollController!.signal,
    });
}

Ensures only one active scroll listener exists at any time, preventing accumulation across Astro page transitions.

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