deleteConversation method
Deletes a conversation by its ID.
If the deleted conversation was the current one, the first remaining conversation becomes current, or null if none remain.
Implementation
void deleteConversation(String conversationId) {
_conversations.removeWhere((conv) => conv.id == conversationId);
if (_currentConversation?.id == conversationId) {
_currentConversation =
_conversations.isNotEmpty ? _conversations.first : null;
}
notifyListeners();
}