Add stale cache for public data

src/api.ts

@@ -12,10 +12,59 @@ export function assetUrl(path: string | undefined | null) {
   return `${apiBase}${path}`;
 }
 
+type JsonCacheEntry<T> = {
+  cachedAt: number;
+  value: T;
+};
+
+const jsonMemoryCache = new Map<string, JsonCacheEntry<unknown>>();
+const jsonStoragePrefix = "ark-json-cache:v1:";
+
+function jsonCacheKey(path: string): string {
+  return `${apiBase}${path}`;
+}
+
+export function readJSONCache<T>(path: string): T | null {
+  const key = jsonCacheKey(path);
+  const memoryEntry = jsonMemoryCache.get(key) as JsonCacheEntry<T> | undefined;
+  if (memoryEntry) return memoryEntry.value;
+
+  if (typeof window === "undefined") return null;
+
+  try {
+    const raw = window.localStorage.getItem(`${jsonStoragePrefix}${key}`);
+    if (!raw) return null;
+    const entry = JSON.parse(raw) as JsonCacheEntry<T>;
+    jsonMemoryCache.set(key, entry as JsonCacheEntry<unknown>);
+    return entry.value;
+  } catch {
+    return null;
+  }
+}
+
+function writeJSONCache<T>(path: string, value: T): void {
+  const key = jsonCacheKey(path);
+  const entry: JsonCacheEntry<T> = { cachedAt: Date.now(), value };
+  jsonMemoryCache.set(key, entry as JsonCacheEntry<unknown>);
+
+  if (typeof window === "undefined") return;
+
+  try {
+    window.localStorage.setItem(
+      `${jsonStoragePrefix}${key}`,
+      JSON.stringify(entry),
+    );
+  } catch {
+    // Ignore storage quota / privacy-mode failures; memory cache still works.
+  }
+}
+
 export async function getJSON<T>(path: string): Promise<T> {
   const res = await fetch(`${apiBase}${path}`);
   if (!res.ok) throw new Error(await res.text());
-  return res.json() as Promise<T>;
+  const data = (await res.json()) as T;
+  writeJSONCache(path, data);
+  return data;
 }
 
 export async function getJSONAuth<T>(path: string, token: string): Promise<T> {