Refactor: extract duplicated menu dropdown code to shared module

[Copilot]

The menu dropdown initialization and click-outside handler were duplicated across index.html and books.html. Extracted to a shared module to eliminate 44 lines of duplication.

Changes

• Created js/menu.js: Centralized toggleMenu() function and click-outside event listener
• Updated index.html and books.html: Replaced inline implementations with <script src="js/menu.js"></script>

Implementation

// js/menu.js
function toggleMenu() {
    const dropdown = document.querySelector('.menu-dropdown');
    const btn = document.querySelector('.menu-button');
    const willShow = !dropdown.classList.contains('show');
    dropdown.classList.toggle('show');
    if (btn) btn.setAttribute('aria-expanded', willShow ? 'true' : 'false');
}

document.addEventListener('click', function(event) {
    const menuWrapper = document.querySelector('.menu-wrapper');
    if (menuWrapper && !menuWrapper.contains(event.target)) {
        const dropdown = document.querySelector('.menu-dropdown');
        if (dropdown) {
            dropdown.classList.remove('show');
            const btn = document.querySelector('.menu-button');
            if (btn) btn.setAttribute('aria-expanded', 'false');
        }
    }
});

Screenshot

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