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/build.gradle

@@ -0,0 +1,25 @@
+plugins {
+  id 'com.android.library'
+  id 'expo-module-gradle-plugin'
+}
+
+group = 'tech.chrisedwards.musicshare.push'
+version = '0.1.0'
+
+android {
+  namespace "tech.chrisedwards.musicshare.push"
+  defaultConfig {
+    versionCode 1
+    versionName "0.1.0"
+  }
+  lintOptions {
+    abortOnError false
+  }
+}
+
+dependencies {
+  // UnifiedPush: notifications through a distributor app the user picks (e.g. ntfy on our own
+  // server) instead of Google's Firebase. Open source (Apache-2.0), no Play Services.
+  implementation "org.unifiedpush.android:connector:3.3.5"
+  implementation "androidx.core:core-ktx:1.16.0"
+}