Refactor: extract shared JavaScript to common.js

[Copilot]

Eliminates code duplication between index.html and books.html by consolidating shared JavaScript functions into a single file.

Changes

• Created js/common.js: Consolidates theme toggle (toggleTheme()), menu dropdown (toggleMenu()), and click-outside handler previously duplicated across both HTML files
• Updated index.html and books.html: Replaced inline <script> blocks with <script src="js/common.js"></script>
• Preserved page-specific logic: Book cover loading and ISBN helper functions remain in books.html

Impact

• Removes 90 lines of duplicate code
• Single source of truth for shared UI interactions
• Future changes to theme/menu behavior require updates in one location

Example

Before:

<!-- index.html -->
<script>
  const currentTheme = localStorage.getItem('theme') || 'light';
  document.documentElement.setAttribute('data-theme', currentTheme);
  function toggleTheme() { /* ... */ }
  function toggleMenu() { /* ... */ }
  // ... 40+ more lines
</script>

<!-- books.html -->
<script>
  const currentTheme = localStorage.getItem('theme') || 'light';
  document.documentElement.setAttribute('data-theme', currentTheme);
  function toggleTheme() { /* ... */ }
  function toggleMenu() { /* ... */ }
  // ... 40+ more lines
</script>

After:

<!-- Both files -->
<script src="js/common.js"></script>

Screenshots

Theme toggle and menu functionality verified on both pages:

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