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>

src/ui/screens.ts

@@ -79,9 +79,11 @@ export function initLessonScreen(
 	level: number,
 	layout: KeyboardLayout,
 	onComplete: (level: number) => void,
+	onCancel?: () => void,
 ): () => void {
 	const letterArea = document.getElementById("letter-area")!;
 	const keyboardContainer = document.getElementById("lesson-keyboard")!;
+	const backBtn = document.getElementById("lesson-back-btn") as HTMLButtonElement;
 	const state = createExerciseState(level);
 
 	// Render keyboard
@@ -131,6 +133,13 @@ export function initLessonScreen(
 
 	document.addEventListener("keydown", onKeyDown);
 
+	// Back button to cancel lesson
+	backBtn.onclick = () => {
+		document.removeEventListener("keydown", onKeyDown);
+		letterArea.innerHTML = "";
+		if (onCancel) onCancel();
+	};
+
 	// Return cleanup function
 	return () => {
 		document.removeEventListener("keydown", onKeyDown);