feat: implement section-based color coding

- Add CSS variables for section colors (CSS violet, HTML raspberry, Tailwind cyan)
- Set data-section attribute on body based on current route
- Color code header nav links, logo, reference nav, topic links
- Color code CodeMirror editor cursor/selection
- Color code task instruction bubble
- Add reference page footer
- Section pages, lessons, and references all use matching colors

src/index.html

@@ -243,6 +243,9 @@
 						<!-- Reference content injected by JS -->
 					</div>
 				</article>
+				<footer class="reference-footer">
+					<p>&copy; 2025 <a href="https://librete.ch">LibreTECH</a>. <span data-i18n="footerLicense">Released into public domain.</span></p>
+				</footer>
 			</div>
 
 			<main class="game-layout" id="main-content">