Notifications that arrive when ihasmail is closed

ihasmail's notifications came from EventSource, which lives exactly as
long as a tab does -- so "desktop notifications" has always quietly
meant "while you are looking". That switch is now labelled as much, and
a second one does the thing people assumed the first one did.

Stalwart 0.16 signs Web Push with VAPID (RFC 9749) and can put the
message itself in the payload (draft-ietf-jmap-emailpush). The server
pushes straight to the browser's own push service: ihasmail's server is
not in the delivery path, there is no relay to run, and nothing beyond
the browser vendor's endpoint that Web Push requires of everyone.

Checked against the live 0.16.19 before any of this was written, because
an advertised capability is not a configured one:

  - the session publishes a real applicationServerKey, so no key
    generation or server configuration is needed
  - PushSubscription/get answers an ordinary user rather than refusing
  - emailpush is advertised, and its draft defines a filter, an ordered
    properties list and an urgency -- so the payload can carry sender and
    subject, and the server drops properties from the end when it will
    not fit rather than failing the notification

Three things this gets right that are easy to get wrong:

  - The verification handshake. A JMAP subscription delivers nothing
    until the client echoes back a code the server pushed, and the
    service worker cannot answer it -- no credentials in that context.
    It forwards the code to a tab, or leaves it in the cache when no tab
    was open to forward it to.

  - Key encoding. The W3C Push API produces unpadded base64url and
    Stalwart 0.16 was fixed to accept exactly that, so nothing here pads
    on the way out. The VAPID key needs padding on the way *in* for
    atob; getting that backwards fails at subscribe() with an opaque
    error, so it lives in one named function with tests.

  - Sign-out. A subscription belongs to the account, not the session.
    Without tearing it down, a shared machine keeps notifying for a
    mailbox nobody is signed into -- which is somebody else's mail.

The mock models the JMAP half, including refusing padded keys and
non-https endpoints, and creating subscriptions *unverified*. Delivery
cannot be mocked -- it runs through the browser vendor's real push
service -- but a mock that marked a subscription verified on creation
would let a client ship without the handshake, and the symptom in
production is "registered, and silent".

Not verified end to end: an actual notification arriving. That needs a
real browser, a real push service and real delivery, so it is live
testing or nothing.

web/src/lib/webpushEnable.ts

@@ -0,0 +1,103 @@
+/**
+ * Turning Web Push on and off, and completing the handshake it needs.
+ *
+ * Kept apart from `webpush.ts` so that module stays pure JMAP and stays
+ * testable: everything here touches the browser's service worker and
+ * permission prompt, none of which exists under a test runner.
+ */
+import { CAP } from "@/jmap/client";
+import { useSession } from "@/store/session";
+import {
+  applicationServerKey,
+  createSubscription,
+  decodeApplicationServerKey,
+  listSubscriptions,
+  subscriptionPayload,
+  unsubscribeThisDevice,
+  verifySubscription,
+  webPushAvailable,
+} from "@/lib/webpush";
+
+let listening = false;
+
+/**
+ * Watch for the verification code the server pushes.
+ *
+ * The service worker cannot answer it — a JMAP call needs the session cookie
+ * and this is a background context — so it forwards the code here, or leaves it
+ * in the cache when no tab was open to forward it to.
+ */
+export function listenForVerification(): void {
+  if (listening || typeof navigator === "undefined" || !("serviceWorker" in navigator)) return;
+  listening = true;
+  navigator.serviceWorker.addEventListener("message", (e: MessageEvent) => {
+    const d = e.data as { type?: string; id?: string; code?: string } | undefined;
+    if (d?.type === "push-verification" && d.id && d.code) void verifySubscription(d.id, d.code).catch(() => {});
+  });
+  void collectStoredVerification();
+}
+
+/** Pick up a code that arrived while no tab was open. */
+async function collectStoredVerification(): Promise<void> {
+  try {
+    const cache = await caches.open("ihasmail-v2");
+    const hit = await cache.match("ihasmail-push-verification");
+    if (!hit) return;
+    const { id, code } = (await hit.json()) as { id?: string; code?: string };
+    await cache.delete("ihasmail-push-verification");
+    if (id && code) await verifySubscription(id, code);
+  } catch {
+    /* nothing waiting, or no cache: not a failure */
+  }
+}
+
+/**
+ * Subscribe this browser. Safe to call again — the deviceClientId makes a
+ * repeat replace rather than accumulate.
+ *
+ * Returns why it could not, rather than throwing, because every reason is
+ * something to tell the user plainly: an old server, a browser without push, a
+ * permission they declined.
+ */
+export async function enableWebPush(): Promise<{ ok: true } | { ok: false; reason: string }> {
+  if (!webPushAvailable()) {
+    return { ok: false, reason: "This browser or mail server does not support background notifications." };
+  }
+  if (Notification.permission === "denied") {
+    return { ok: false, reason: "Notifications are blocked for this site in your browser's settings." };
+  }
+  const key = applicationServerKey();
+  if (!key) return { ok: false, reason: "This mail server does not publish a push key." };
+
+  try {
+    const reg = await navigator.serviceWorker.ready;
+    const existing = await reg.pushManager.getSubscription();
+    const sub = existing ?? (await reg.pushManager.subscribe({
+      // Web Push requires it, and Chrome refuses a subscription without it.
+      userVisibleOnly: true,
+      applicationServerKey: decodeApplicationServerKey(key),
+    }));
+    const accountId = useSession.getState().accountFor(CAP.mail);
+    await createSubscription(subscriptionPayload(sub, accountId));
+    listenForVerification();
+    return { ok: true };
+  } catch (err) {
+    return { ok: false, reason: (err as Error).message || "Could not subscribe to notifications." };
+  }
+}
+
+/** Remove this browser's subscription, at the browser and at the server. */
+export async function disableWebPush(): Promise<void> {
+  await unsubscribeThisDevice();
+}
+
+/** Whether this browser currently has a verified subscription registered. */
+export async function webPushActive(): Promise<boolean> {
+  try {
+    const reg = await navigator.serviceWorker?.getRegistration();
+    if (!(await reg?.pushManager.getSubscription())) return false;
+    return (await listSubscriptions()).length > 0;
+  } catch {
+    return false;
+  }
+}