Add service worker implementation and tests for PWA caching strategy

- Implemented a service worker (sw.js) that handles installation, activation, and fetch events.
- Added caching logic for public assets and a fallback for offline navigation.
- Created unit tests (sw.test.ts) to validate service worker behavior, including asset caching, navigation handling, and activation logic.
- Ensured that private API routes and third-party requests are excluded from caching.

src/pwa/client.ts

@@ -0,0 +1,90 @@
+import { useSyncExternalStore } from "react";
+
+type InstallPrompt = Event & {
+  prompt: () => Promise<void>;
+  userChoice: Promise<{ outcome: "accepted" | "dismissed" }>;
+};
+type PwaState = { enabled: boolean; installed: boolean; installPrompt: InstallPrompt | null; updateReady: boolean; error: string };
+let state: PwaState = { enabled: false, installed: false, installPrompt: null, updateReady: false, error: "" };
+const listeners = new Set<() => void>();
+let registration: ServiceWorkerRegistration | undefined;
+let reloading = false;
+let started = false;
+const publish = (change: Partial<PwaState>) => {
+  state = { ...state, ...change };
+  listeners.forEach(listener => listener());
+};
+const subscribe = (listener: () => void) => { listeners.add(listener); return () => { listeners.delete(listener); }; };
+export const usePwa = () => useSyncExternalStore(subscribe, () => state, () => state);
+const subscribeOnline = (listener: () => void) => {
+  window.addEventListener("online", listener);
+  window.addEventListener("offline", listener);
+  return () => { window.removeEventListener("online", listener); window.removeEventListener("offline", listener); };
+};
+export const useOnline = () => useSyncExternalStore(subscribeOnline, () => navigator.onLine !== false, () => true);
+
+/** Called once before React renders so early install events cannot be missed. */
+export function startPwa() {
+  if (started || !window.isSecureContext || !("serviceWorker" in navigator)) return;
+  started = true;
+  const standalone = window.matchMedia("(display-mode: standalone)");
+  const installed = () => standalone.matches || (navigator as Navigator & { standalone?: boolean }).standalone === true;
+  publish({ enabled: true, installed: installed() });
+  standalone.addEventListener("change", () => publish({ installed: installed() }));
+  window.addEventListener("beforeinstallprompt", event => {
+    event.preventDefault();
+    publish({ installPrompt: event as InstallPrompt });
+  });
+  window.addEventListener("appinstalled", () => publish({ installed: true, installPrompt: null }));
+  let controller = navigator.serviceWorker.controller;
+  navigator.serviceWorker.addEventListener("controllerchange", () => {
+    if (reloading) window.location.reload();
+    else if (controller) publish({ updateReady: true });
+    controller = navigator.serviceWorker.controller;
+  });
+  const register = async () => {
+    try {
+      registration = await navigator.serviceWorker.register("/sw.js", { scope: "/", updateViaCache: "none" });
+      publish({ updateReady: Boolean(registration.waiting) });
+      const observeInstall = () => {
+        const worker = registration?.installing;
+        worker?.addEventListener("statechange", () => {
+          if (worker.state === "installed") publish({ updateReady: Boolean(navigator.serviceWorker.controller), error: "" });
+          if (worker.state === "redundant" && !navigator.serviceWorker.controller)
+            publish({ error: "Offline setup didn’t finish. Reconnect and reload to try again." });
+        });
+      };
+      registration.addEventListener("updatefound", observeInstall);
+      observeInstall();
+      const check = () => {
+        if (navigator.onLine && document.visibilityState === "visible") void registration?.update().catch(() => {});
+      };
+      document.addEventListener("visibilitychange", check);
+      window.addEventListener("online", check);
+      window.setInterval(check, 60 * 60 * 1000);
+    } catch {
+      publish({ error: "Offline setup is unavailable. Reconnect and reload to try again." });
+    }
+  };
+  if (document.readyState === "complete") void register();
+  else window.addEventListener("load", () => { void register(); }, { once: true });
+}
+
+export async function installApp(): Promise<boolean> {
+  const prompt = state.installPrompt;
+  if (!prompt) return false;
+  publish({ installPrompt: null });
+  try {
+    await prompt.prompt();
+    await prompt.userChoice;
+  } catch { publish({ error: "Could not open installation. Try your browser’s install menu." }); }
+  return true;
+}
+
+export function applyUpdate() {
+  if (!state.updateReady) return;
+  if (registration?.waiting) {
+    reloading = true;
+    registration.waiting.postMessage({ type: "ACTIVATE_UPDATE" });
+  } else window.location.reload();
+}