fix(security): treat a new identity key as a message request, never as a known contact

A stranger could pick the display name of an existing contact ("Mom")
and appear as that contact. Messages from a key that is not in the
contact list now open a message request labelled "(unknown)": no
replies, read receipts, typing indicators, calls or name in
notifications until the user accepts it and names the contact
themselves. Names that look like an existing contact (case, diacritics,
Cyrillic/Greek homoglyphs, invisible characters) also show a warning in
the real contact's chat.

When the relay account of a known contact shows up with a different
key, the chat shows "safety number changed", verification is cleared
and the session that now belongs to the other key is dropped before
sending, so a reply never goes to the new key.

app/lib/providers/conversations_provider.dart

@@ -341,8 +341,10 @@ class ConversationsNotifier extends AsyncNotifier<List<Conversation>> {
 
     // Příchozí zprávy, které jsem právě přečetl → pošlu za ně potvrzení
     // přečtení (sendReadReceipts sám zkontroluje, jestli je funkce zapnutá).
+    // Upozornění appky (notice) nikdo neposlal — potvrzovat ho nelze.
     final justRead = conv.messages
-        .where((m) => !m.isMine(identity.id) && !m.isRead)
+        .where((m) =>
+            !m.isMine(identity.id) && !m.isRead && m.notice == null)
         .toList();
 
     conv.markAllRead(identity.id);