Update prompt and offline indicator (#52)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>

web/src/pwa/networkStatus.ts

@@ -0,0 +1,50 @@
+// NetworkWindow is the part of window the network status reads: the browser's
+// online flag and its online/offline events.
+export type NetworkWindow = Pick<Window, 'navigator' | 'addEventListener' | 'removeEventListener'>
+
+export type NetworkStatus = {
+  // offline reports whether the browser says it is offline or the last API
+  // request failed with a network error.
+  offline(): boolean
+  // subscribe is notified when offline may have changed. It listens for the
+  // window's online/offline events until the returned function is called.
+  subscribe(listener: () => void): () => void
+  // requestFailed records a fetch that rejected with a network error.
+  requestFailed(): void
+  // requestSucceeded records a fetch that got a response.
+  requestSucceeded(): void
+}
+
+// createNetworkStatus tracks whether MealQuest can reach the server. A failed
+// request counts as offline until a later request gets a response or the
+// browser reports that it is back online.
+export function createNetworkStatus(win: NetworkWindow): NetworkStatus {
+  let failed = false
+  const listeners = new Set<() => void>()
+
+  function setFailed(value: boolean) {
+    if (failed === value) return
+    failed = value
+    listeners.forEach((listener) => listener())
+  }
+
+  return {
+    offline: () => !win.navigator.onLine || failed,
+    subscribe(listener) {
+      const online = () => {
+        failed = false
+        listener()
+      }
+      listeners.add(listener)
+      win.addEventListener('online', online)
+      win.addEventListener('offline', listener)
+      return () => {
+        listeners.delete(listener)
+        win.removeEventListener('online', online)
+        win.removeEventListener('offline', listener)
+      }
+    },
+    requestFailed: () => setFailed(true),
+    requestSucceeded: () => setFailed(false),
+  }
+}