feat(chat): app-wide inbox listener so messages arrive without opening the chat

Reception only ran inside an open ChatScreen for a known peer, so a first
message from a new peer was invisible: nothing in local storage -> nothing
in the inbox list -> you never opened the chat -> you never subscribed.

Add InboxService: one long-lived NIP-17 inbox subscription for the whole
app (foreground), persisting every incoming message to MessageStore and
firing a 'changes' signal the inbox list live-reloads on. Reconnects when
the network returns; degrades to nothing offline. Started from main when a
social identity exists.

Make MessageStore.append idempotent (dedup by sender+timestamp+text) and
serialized behind a write lock — the global listener and an open chat's own
subscription now write the same conversation concurrently and relays
re-deliver stored gift wraps on every resubscribe. Tests for both.

Known trade-offs (follow-ups): foreground-only (no push yet); each of
InboxService/ChatScreen/MarketScreen opens its own RelayPool (a shared
connection is a later optimization).

apps/app_seeds/test/services/message_store_test.dart

@@ -46,6 +46,19 @@ void main() {
     expect(convos.last.lastText, 'back A');
   });
 
+  test('append is idempotent: a re-delivered message is not duplicated',
+      () async {
+    // Same sender + timestamp + text = the same gift wrap redelivered by a
+    // relay on resubscribe. It must not pile up.
+    expect(await store.append('peer', msg('peer', 'hola', 1000)), isTrue);
+    expect(await store.append('peer', msg('peer', 'hola', 1000)), isFalse);
+    expect(await store.history('peer'), hasLength(1));
+
+    // A genuinely different message (later timestamp) still lands.
+    expect(await store.append('peer', msg('peer', 'hola', 2000)), isTrue);
+    expect(await store.history('peer'), hasLength(2));
+  });
+
   test('history is capped to the most recent 200', () async {
     for (var i = 0; i < 210; i++) {
       await store.append('peer', msg('me', 'm$i', i));