feat(messages): unread badges + OS notification for private messages

Private messages (NIP-17) arrived in the foreground with no signal: no
unread count, no badge, no OS notification. Hook both into the single
InboxService.ingest choke point.

- UnreadService: per-peer "last read" in the keystore (timestamps+pubkey
  only, no message text), live count via a changes stream, active-peer
  suppression so the open chat never badges or notifies.
- UnreadBadge: reusable, RTL-aware (Flutter Badge) — on the home hamburger,
  the drawer "Chat" item, and per-conversation in the messages list.
- Mark-read on chat open (ChatScreen).
- NotificationService over flutter_local_notifications: generic
  "New message from <name>" (no message text, for privacy), payload = pubkey,
  tap opens /chat/:pubkey. No-op on web/Windows; Android POST_NOTIFICATIONS.
- i18n notifications.newMessageFrom (en/es/pt/ast).

Background/push stays out of scope (foreground-only by design).

Tests: UnreadService, NotificationService (mock plugin), InboxService hooks
(new peer notifies/counts; duplicate, own, and open-chat do not), and a
UnreadBadge widget test. dart analyze clean; commons_core green.

apps/app_seeds/lib/services/unread_service.dart

@@ -0,0 +1,96 @@
+import 'dart:async';
+
+import 'package:commons_core/commons_core.dart';
+
+import '../security/secret_store.dart';
+import 'message_store.dart';
+
+/// Tracks which private messages are still unread, so the UI can show a badge.
+///
+/// There is no read/unread flag on a message; instead we keep a per-peer
+/// "last read at" timestamp in the keystore (just a timestamp and a pubkey — no
+/// message text, so it honours "no plaintext at rest"). A conversation's unread
+/// count is simply the peer's messages newer than that mark. Reading history
+/// from the [MessageStore] keeps a single source of truth.
+///
+/// Only inbound messages count: a message whose author is the peer. Your own
+/// sent messages (stored with your pubkey) are never counted.
+///
+/// [activePeer] is the chat currently on screen. A message that arrives for it
+/// is treated as already read (you're looking at it), and the inbox listener
+/// also skips its notification — so the chat you're in never badges or beeps.
+class UnreadService {
+  UnreadService(this._store, this._secrets);
+
+  final MessageStore _store;
+  final SecretStore _secrets;
+
+  final _changes = StreamController<void>.broadcast();
+
+  /// The peer whose chat is currently open, or null. Set by [ChatScreen].
+  String? activePeer;
+
+  static const _prefix = 'tane.social.unread.';
+
+  String _key(String peer) => '$_prefix$peer';
+
+  /// Fires (no payload) whenever an unread count may have changed — on a new
+  /// inbound message and on [markRead]. Broadcast, so several badges can listen.
+  Stream<void> get changes => _changes.stream;
+
+  /// Records that a new [message] for [peer] was just persisted. If its chat is
+  /// open it is marked read immediately; otherwise badges are asked to refresh.
+  Future<void> onMessageReceived(String peer, PrivateMessage message) async {
+    if (peer == activePeer) {
+      await markRead(peer);
+    } else {
+      _fire();
+    }
+  }
+
+  /// Marks the conversation with [peer] read up to its latest message, so its
+  /// unread count drops to zero. Called when the chat opens (and when a message
+  /// arrives while it's open).
+  Future<void> markRead(String peer) async {
+    final history = await _store.history(peer);
+    final upTo = history.isEmpty
+        ? DateTime.now()
+        : history
+            .map((m) => m.at)
+            .reduce((a, b) => a.isAfter(b) ? a : b);
+    await _secrets.write(_key(peer), '${upTo.millisecondsSinceEpoch}');
+    _fire();
+  }
+
+  /// Unread inbound messages from [peer] (those newer than its last-read mark).
+  Future<int> unreadCount(String peer) async {
+    final lastRead = await _lastRead(peer);
+    final history = await _store.history(peer);
+    return history
+        .where((m) => m.fromPubkey == peer && m.at.isAfter(lastRead))
+        .length;
+  }
+
+  /// Unread messages across every conversation (the number for the app badge).
+  Future<int> totalUnreadCount() async {
+    var total = 0;
+    for (final c in await _store.conversations()) {
+      total += await unreadCount(c.peerPubkey);
+    }
+    return total;
+  }
+
+  Future<DateTime> _lastRead(String peer) async {
+    final raw = await _secrets.read(_key(peer));
+    final ms = raw == null ? null : int.tryParse(raw);
+    return DateTime.fromMillisecondsSinceEpoch(ms ?? 0);
+  }
+
+  void _fire() {
+    if (!_changes.isClosed) _changes.add(null);
+  }
+
+  Future<void> dispose() async {
+    if (!_changes.isClosed) await _changes.close();
+  }
+}