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/core/storage/secure_storage.dart

@@ -442,11 +442,15 @@ class SecureStorageService {
     );
   }
 
-  Future<void> addMessage(String contactId, Message message) async {
+  /// Přidá zprávu do konverzace. Vrací false, když už tam zpráva s tímhle id
+  /// je (opakované doručení) — volající pak přeskočí navazující kroky jako
+  /// doručenku nebo stahování přílohy.
+  Future<bool> addMessage(String contactId, Message message) async {
     var conv = await loadConversation(contactId);
     conv ??= Conversation(contactId: contactId);
-    conv.addMessage(message);
+    if (!conv.addMessage(message)) return false;
     await saveConversation(conv);
+    return true;
   }
 
   /// Založí prázdnou konverzaci, pokud ještě není (nová skupina musí být