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/push/push_notifier.dart

@@ -75,6 +75,19 @@ class PushNotifier {
     );
   }
 
+  /// Zpráva od neznámého odesílatele (žádost o zprávu): bez jména, které
+  /// si zvolil sám.
+  static Future<void> showMessageRequestNotification() async {
+    await _ensureInit();
+    final l10n = l10nFor(await LocaleNotifier.readStored());
+    await _plugin.show(
+      _notificationId,
+      l10n.commonAppName,
+      l10n.pushMessageRequest,
+      _details(l10n),
+    );
+  }
+
   /// Generická notifikace bez obsahu. Klepnutí otevře appku (unlock →
   /// WS připojení → fronta se vyleje normální cestou).
   static Future<void> showWakeNotification() async {