feat: add Reference link to main navigation

- Add Reference nav link with subtle separator styling
- Update nav highlight to show active state on reference pages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

src/index.html

@@ -31,6 +31,7 @@
 						<a href="#css" class="nav-link" data-section="css">CSS</a>
 						<a href="#html" class="nav-link" data-section="html">HTML</a>
 						<a href="#tailwind" class="nav-link" data-section="tailwind">Tailwind</a>
+						<a href="#reference/css" class="nav-link nav-link-ref" data-section="reference">Reference</a>
 					</nav>
 					<button id="help-btn" class="help-toggle" data-i18n-aria-label="help" aria-label="Help">?</button>
 				</div>