fix(01): add lesson cancel button and handle level 6 completion

- Back button in lesson screen to return to forest without completing
- "Alle Stufen geschafft!" message when all 6 levels are done
- Cap currentLevel at MAX_LEVEL to prevent out-of-bounds progression

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

index.html

@@ -38,6 +38,7 @@
 		</section>
 		<section id="screen-lesson" class="screen">
 			<div class="lesson">
+				<button class="lesson__back-btn" id="lesson-back-btn">&#x2190; Zurueck</button>
 				<div class="lesson__letter-area" id="letter-area">
 					<!-- Current letter appears here -->
 				</div>