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/core/l10n/contact_name.dart

@@ -0,0 +1,12 @@
+import 'package:sepot/l10n/generated/app_localizations.dart';
+import 'package:sepot/models/contact.dart';
+
+/// Jméno kontaktu pro UI. U žádosti o zprávu s viditelnou značkou, že si
+/// jméno zvolil odesílatel sám: „Máma (neznámý)“ se nesmí na pohled
+/// shodovat s přijatou „Mámou“ v seznamu, hlavičce ani ve skupině.
+String contactName(AppLocalizations l10n, Contact contact) {
+  if (contact.displayName.isEmpty) return l10n.commonUnknownContact;
+  return contact.accepted
+      ? contact.displayName
+      : l10n.contactRequestName(contact.displayName);
+}