fix(app): ping the relay, retry stuck messages, drop duplicates

A message sent into a dead socket ended up as failed — attachments even
stayed on "sending" forever — and only the user could send it again by
hand. Mobile NAT drops idle TCP silently, so neither side knew.

- pingInterval on the WebSocket, so the client notices a dead link itself
- outbox: after every reconnect, stuck messages are re-sent (batch of 30,
  up to a day old); permanent errors are not retried
- sendAttachment gained the try/catch it was missing, otherwise the
  attachment never reached the outbox
- de-duplicate by message id: a retry re-sends the SAME message, which
  would otherwise show up twice on the other side

app/lib/models/conversation.dart

@@ -39,10 +39,16 @@ class Conversation {
         .length;
   }
 
-  /// Přidej zprávu
-  void addMessage(Message message) {
+  /// Přidej zprávu. Vrací false, když už zpráva s tímhle id v konverzaci je.
+  ///
+  /// Id zprávy nese odesílatel (pole `id` na drátě), takže opakované doručení
+  /// téhož obsahu — retry odesílatele po výpadku spojení nebo redelivery
+  /// z fronty relaye — nesmí založit druhou bublinu.
+  bool addMessage(Message message) {
+    if (messages.any((m) => m.id == message.id)) return false;
     messages.add(message);
     lastActivity = DateTime.now();
+    return true;
   }
 
   /// Nahraď zprávu se stejným id (změna stavu přílohy, doručenka…).