Consolidate duplicate theme toggle code into shared JS module

[Copilot]

The theme toggle implementation was duplicated across index.html (lines 79-100) and books.html (lines 51-65, 188-191), violating DRY principles.

Changes

• Created js/theme.js: Extracted theme initialization, toggle function, and icon state management into shared module
• Updated both HTML files: Removed inline theme code, added <script src="js/theme.js"></script> reference

Result

Net -17 lines across codebase. Theme logic now lives in single source of truth, simplifying future maintenance.

// js/theme.js - now powers both pages
function toggleTheme() {
    const theme = document.documentElement.getAttribute('data-theme');
    const newTheme = theme === 'light' ? 'dark' : 'light';
    document.documentElement.setAttribute('data-theme', newTheme);
    localStorage.setItem('theme', newTheme);
    // Update icon...
}

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