Add folder game, file browser UI, and automated tests.

Introduces a localStorage-backed messy-desktop challenge with sub-steps,
drag-and-drop and Places/Back navigation, IBM Plex Mono, and a /api/health
endpoint. Adds Vitest coverage for the API and level logic plus Playwright
smoke tests.

Made-with: Cursor

src/lib/folder-game/storage.ts

@@ -0,0 +1,57 @@
+import type { SaveV1 } from "./types";
+
+import {
+  LAST_PLAYABLE,
+  freshSave,
+  migrateSubIndex,
+  subCount,
+} from "./levels";
+
+export const STORAGE_KEY = "folder-game-challenge:v1";
+
+export function parseSave(raw: string | null): SaveV1 | null {
+  if (!raw) return null;
+  try {
+    const v = JSON.parse(raw) as SaveV1;
+    if (v?.version !== 1) return null;
+    if (typeof v.levelIndex !== "number") return null;
+    if (v.levelIndex < 0 || v.levelIndex > LAST_PLAYABLE + 1) return null;
+    if (!v.fs?.nodes || typeof v.fs.viewParentId !== "string") return null;
+    if (!Array.isArray(v.fs.visitedFolderIds)) return null;
+    const subIndex = migrateSubIndex(
+      v.levelIndex,
+      v.fs,
+      typeof v.subIndex === "number" ? v.subIndex : undefined,
+    );
+    const maxSub = Math.max(0, subCount(v.levelIndex) - 1);
+    return { ...v, subIndex: Math.min(subIndex, maxSub) };
+  } catch {
+    return null;
+  }
+}
+
+export function loadSave(): SaveV1 | null {
+  if (typeof window === "undefined") return null;
+  return parseSave(localStorage.getItem(STORAGE_KEY));
+}
+
+export function writeSave(save: SaveV1) {
+  localStorage.setItem(STORAGE_KEY, JSON.stringify(save));
+}
+
+export function clearSave() {
+  localStorage.removeItem(STORAGE_KEY);
+}
+
+export function resumeOrNew(): SaveV1 {
+  return loadSave() ?? freshSave(0);
+}
+
+export function hasInProgressSave(): boolean {
+  const s = loadSave();
+  return s !== null && s.levelIndex <= LAST_PLAYABLE;
+}
+
+export function isFinished(save: SaveV1) {
+  return save.levelIndex > LAST_PLAYABLE;
+}