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/app.ts

@@ -20,6 +20,8 @@ export function getDB(): IDBDatabase {
 	return db;
 }
 
+const MAX_LEVEL = 6;
+
 async function handleLessonComplete(completedLevel: number): Promise<void> {
 	// Update progress after lesson completion
 	const progress = await getProgress(db);
@@ -31,7 +33,8 @@ async function handleLessonComplete(completedLevel: number): Promise<void> {
 	}
 
 	// Advance to next level if this was the current level (LEVL-02, LEVL-03)
-	if (completedLevel === progress.currentLevel) {
+	// Cap at MAX_LEVEL + 1 so we don't go beyond defined levels
+	if (completedLevel === progress.currentLevel && completedLevel < MAX_LEVEL) {
 		progress.currentLevel = completedLevel + 1;
 	}
 
@@ -51,6 +54,11 @@ async function handleLessonComplete(completedLevel: number): Promise<void> {
 	showScreen("forest");
 }
 
+async function handleLessonCancel(): Promise<void> {
+	await initForestScreen(db, startLessonFromForest);
+	showScreen("forest");
+}
+
 function startLessonFromForest(level: number): void {
 	// Clean up previous lesson if any
 	if (lessonCleanup) {
@@ -69,6 +77,7 @@ function startLessonFromForest(level: number): void {
 			level,
 			layout,
 			handleLessonComplete,
+			handleLessonCancel,
 		);
 	});
 }