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/contacts_provider.dart

@@ -26,9 +26,19 @@ class ContactsNotifier extends AsyncNotifier<List<Contact>> {
       throw const ContactLinkIncompleteError();
     }
 
-    // Kontrola duplicity
-    if (contacts.any((c) => c.publicIdentity.id == identity.id)) {
-      throw const ContactAlreadyExistsError();
+    // Kontrola duplicity. Výjimka: žádost o zprávu od téhož klíče — ruční
+    // přidání (QR, odkaz) je právě to přijetí, na které čekala.
+    final existing =
+        contacts.where((c) => c.publicIdentity.id == identity.id).firstOrNull;
+    if (existing != null) {
+      if (existing.accepted) throw const ContactAlreadyExistsError();
+      await accept(existing.id,
+          name: identity.displayName.isEmpty
+              ? existing.displayName
+              : identity.displayName);
+      if (verified) existing.verified = true;
+      await storage.saveContact(existing);
+      return existing;
     }
 
     final contact = Contact.fromPublicIdentity(identity, verified: verified);
@@ -38,6 +48,20 @@ class ContactsNotifier extends AsyncNotifier<List<Contact>> {
     return contact;
   }
 
+  /// Přijme žádost o zprávu pod jménem, které jí dal uživatel (jméno
+  /// zvolené odesílatelem se tím nepřebírá automaticky).
+  Future<void> accept(String contactId, {required String name}) async {
+    final storage = ref.read(storageProvider);
+    final contacts = await future;
+    final contact = contacts.where((c) => c.id == contactId).firstOrNull;
+    if (contact == null) return;
+    contact
+      ..displayName = name.trim()
+      ..accepted = true;
+    await storage.saveContact(contact);
+    state = AsyncData([...contacts]);
+  }
+
   /// Smaž kontakt
   Future<void> removeContact(String contactId) async {
     final storage = ref.read(storageProvider);