Code review feedback: accessibility, performance, and code quality improvements

[Copilot]

Addresses all code review feedback from PR #1, focusing on accessibility enhancements, performance optimizations, and memory management fixes.

Accessibility

• Theme toggle: Added role="switch" and aria-checked attributes, dynamic aria-label updates ("Switch to dark mode" / "Switch to light mode")
• Heading hierarchy: Book titles now use <h3> elements (h1 → h2 → h3 structure)
• Keyboard navigation: Menu returns focus to button on Escape, handles null relatedTarget in focusout events
• Dark mode contrast: Text color changed from #a0a0a0 to #b8b8b8 for WCAG AA compliance (4.5:1 ratio)

Performance

• Scripts use defer attribute to prevent blocking HTML parsing
• ISBN cover loading changed from parallel to sequential to ensure deterministic order and prevent race conditions

// Before: All ISBNs probed simultaneously
isbns.forEach(isbn => { /* probe */ });

// After: Sequential probing with deterministic fallback
function tryNextIsbn(index) {
  if (index >= isbns.length) { /* fallback */ }
  // Try next ISBN only on failure
}

Code Quality

• Memory leak fixed: Probe array cleared after cleanup (probes.length = 0)
• 1x1 pixel check uses strict equality (===) instead of <=
• SVG placeholder escaping uses double quotes for consistency
• CSS theme toggle uses custom properties for maintainable calculations
• Color transitions added to nav links for visual consistency
• Console warning when theme icon element missing
• Twitter card changed from summary_large_image to summary (180×180 icon)

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.