feat: add landing pages and section navigation

- Add home landing page with section cards (CSS, HTML, Tailwind)
- Add section landing pages with module grid and progress tracking
- Implement extended URL routing for pages, sections, and lessons
- Create sections.js configuration for module categorization
- Exclude welcome/goodbye modules from progress stats
- Add main navigation links in header (desktop only)
- Update logo click to navigate to home landing

Routes:
- # → Home landing
- #css, #html, #tailwind → Section landing pages
- #module/index → Lesson (unchanged)

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

src/impl/LessonEngine.js

@@ -463,9 +463,12 @@ export class LessonEngine {
 		let totalCompleted = 0;
 
 		this.modules.forEach((module) => {
+			// Skip modules excluded from progress (e.g., welcome, goodbye)
+			if (module.excludeFromProgress) return;
+
 			totalLessons += module.lessons.length;
 			const progress = this.userProgress[module.id];
-			if (progress && progress.completed) {
+			if (progress?.completed) {
 				totalCompleted += progress.completed.length;
 			}
 		});