feat(05-01): extend Progress type with currentForestPage, add paginated renderForestScene

- Add currentForestPage: number field to Progress interface in types.ts
- Export SLOTS_PER_PAGE = 12 constant from scene.ts
- Change renderForestScene signature to accept page: number parameter
- Slice elements by page (start/end = page * SLOTS_PER_PAGE)
- Update navigation UI (forest-nav) after rendering
- Add migrateProgress entry for currentForestPage in app.ts
- Fix Progress object literals in db.test.ts and screens.ts to include new field

src/app.ts

@@ -38,6 +38,7 @@ function migrateProgress(
 	if (!progress.lastPlayedLevels) progress.lastPlayedLevels = [];
 	if (progress.totalCorrect === undefined) progress.totalCorrect = 0;
 	if (progress.totalErrors === undefined) progress.totalErrors = 0;
+	if (progress.currentForestPage === undefined) progress.currentForestPage = 0;
 }
 
 async function handleLessonComplete(
@@ -120,7 +121,9 @@ async function handleLessonComplete(
 		async () => {
 			// "Zurück zum Wald" -- show forest with new element animated
 			await initForestScreen(db, startLessonFromForest);
-			await renderForestScene(db, elementId);
+			const prog = await getProgress(db);
+			const page = prog?.currentForestPage ?? 0;
+			await renderForestScene(db, page, elementId);
 			showScreen("forest");
 		},
 	);