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/features/conversation/chat_actions.dart

@@ -5,6 +5,7 @@ library;
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+import 'package:sepot/core/l10n/contact_name.dart';
 import 'package:sepot/core/l10n/error_messages.dart';
 import 'package:sepot/core/l10n/l10n.dart';
 import 'package:sepot/core/theme/app_theme.dart';
@@ -17,9 +18,7 @@ import 'package:sepot/providers/groups_provider.dart';
 import 'package:sepot/providers/storage_provider.dart';
 
 String contactLabel(BuildContext context, Contact contact) =>
-    contact.displayName.isEmpty
-        ? context.l10n.commonUnknownContact
-        : contact.displayName;
+    contactName(context.l10n, contact);
 
 void _snack(BuildContext context, String text) {
   if (!context.mounted) return;