feat: add module pill indicator and cross-module navigation

- Add module title pill above lesson title showing current category
- Enable next/prev buttons to cross module boundaries
- Automatically advance to next module when completing last lesson
- Go to last lesson of previous module when navigating back

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/index.html

@@ -31,6 +31,7 @@
 				<!-- Left Panel: Instructions + Editor -->
 				<div class="left-panel">
 					<section class="instructions">
+						<span class="module-pill" id="module-pill">Loading...</span>
 						<h2 id="lesson-title">Loading...</h2>
 						<div class="lesson-description" id="lesson-description">
 							Please select a lesson to begin.