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/src/components/Nav.tsx

@@ -4,6 +4,7 @@ import Link from 'next/link';
 import { usePathname } from 'next/navigation';
 import { useState } from 'react';
 import { client, useMe } from '@/lib/api';
+import { ActivityBell } from './ActivityBell';
 
 /** Top bar: logo and the four main places. On phones those move to a bottom tab bar. */
 export function Nav() {
@@ -25,6 +26,7 @@ export function Nav() {
           <img className="brand-logo" src="/logo.png" alt="" width={30} height={30} />
           musicshare
         </Link>
+        {me && <ActivityBell />}
         <div className="nav-links">
           {me ? (
             tabs.map((t) => (