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/forest/scene.ts

@@ -48,6 +48,8 @@ export function renderLevelMap(
 	});
 }
 
+const MAX_LEVEL = 6;
+
 export async function initForestScreen(
 	db: IDBDatabase,
 	onStartLesson: (level: number) => void,
@@ -61,6 +63,19 @@ export async function initForestScreen(
 
 	renderLevelMap(mapContainer, progress, onStartLesson);
 
-	// Continue button starts the current level
-	continueBtn.onclick = () => onStartLesson(progress.currentLevel);
+	const allComplete = levels.every((l) =>
+		progress.completedLevels.includes(l.level),
+	);
+
+	if (allComplete) {
+		continueBtn.textContent = "Alle Stufen geschafft!";
+		continueBtn.disabled = true;
+		continueBtn.classList.add("forest__continue-btn--done");
+	} else {
+		continueBtn.textContent = "Weiter ueben";
+		continueBtn.disabled = false;
+		continueBtn.classList.remove("forest__continue-btn--done");
+		continueBtn.onclick = () =>
+			onStartLesson(Math.min(progress.currentLevel, MAX_LEVEL));
+	}
 }