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/mobile/modules/push/android/src/main/AndroidManifest.xml

@@ -0,0 +1,14 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+  <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+
+  <application>
+    <!-- Receives endpoints and messages from the UnifiedPush distributor (via the connector library). -->
+    <service
+      android:name=".MusicsharePushService"
+      android:exported="false">
+      <intent-filter>
+        <action android:name="org.unifiedpush.android.connector.PUSH_EVENT" />
+      </intent-filter>
+    </service>
+  </application>
+</manifest>