Notifications and sending songs to friends

Reactions to your songs, new followers, and songs sent to you are now
notifications: a bell on the feed (app and website) with an unread
count that updates live, and a list that marks them read. "Send ✉" on
a song sends it to people who follow you, with an optional note.
Settings picks which kinds to be notified about.

Push delivery uses no third-party accounts:
- Android: UnifiedPush through a distributor app (ntfy), pointed at
  our own ntfy server (docker-compose.ntfy.yml). Works the same in
  the F-Droid build; no Firebase.
- Browsers: standard Web Push from the existing service worker.
Both use Web Push encryption with a VAPID key pair the server makes
and keeps in the database. Push addresses are checked like any other
user-supplied URL. Setup is in docs/PUSH.md; the privacy page covers
the new data.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/public/sw.js

@@ -1,8 +1,8 @@
 // musicshare service worker.
 // Deliberately small: live data must always come from the network, so we only
 // cache Next.js's immutable build assets, and show an offline page when a
-// navigation fails. (Web push notifications will hook in here later.)
-const VERSION = 'v1';
+// navigation fails. It also shows push notifications (reactions, follows, songs sent to you).
+const VERSION = 'v2';
 const STATIC_CACHE = `static-${VERSION}`;
 const OFFLINE_URL = '/offline.html';
 
@@ -47,3 +47,35 @@ self.addEventListener('fetch', (event) => {
     event.respondWith(fetch(request).catch(() => caches.match(OFFLINE_URL)));
   }
 });
+
+// Push: { id, title, body, path, icon } from the server, already decrypted by the browser.
+self.addEventListener('push', (event) => {
+  let data = {};
+  try {
+    data = event.data ? event.data.json() : {};
+  } catch {
+    return;
+  }
+  event.waitUntil(
+    self.registration.showNotification(data.title || 'musicshare', {
+      body: data.body || '',
+      icon: data.icon || '/icon-192.png',
+      badge: '/icon-192.png',
+      tag: data.id,
+      data: { path: data.path || '/' },
+    }),
+  );
+});
+
+// Tap: focus an open tab and go there, or open a new one.
+self.addEventListener('notificationclick', (event) => {
+  event.notification.close();
+  const url = new URL(event.notification.data?.path || '/', self.location.origin).href;
+  event.waitUntil(
+    self.clients.matchAll({ type: 'window', includeUncontrolled: true }).then((tabs) => {
+      const tab = tabs.find((t) => new URL(t.url).origin === self.location.origin);
+      if (tab) return tab.focus().then(() => tab.navigate(url));
+      return self.clients.openWindow(url);
+    }),
+  );
+});