feat: Tweak initial info-message for unencrypted chats (#7427)

Fix https://github.com/chatmail/core/issues/7404

src/chat.rs

@@ -3463,7 +3463,13 @@ pub(crate) async fn create_group_ex(
     if !context.get_config_bool(Config::Bot).await?
         && !context.get_config_bool(Config::SkipStartMessages).await?
     {
-        let text = stock_str::new_group_send_first_message(context).await;
+        let text = if !grpid.is_empty() {
+            // Add "Others will only see this group after you sent a first message." message.
+            stock_str::new_group_send_first_message(context).await
+        } else {
+            // Add "Messages in this chat use classic email and are not encrypted." message.
+            stock_str::chat_unencrypted_explanation(context).await
+        };
         add_info_msg(context, chat_id, &text, create_smeared_timestamp(context)).await?;
     }
     if let (true, true) = (sync.into(), !grpid.is_empty()) {

src/stock_str.rs

@@ -447,6 +447,9 @@ https://delta.chat/donate"))]
         fallback = "You are using a proxy. If you're having trouble connecting, try a different proxy."
     ))]
     ProxyEnabledDescription = 221,
+
+    #[strum(props(fallback = "Messages in this chat use classic email and are not encrypted."))]
+    ChatUnencryptedExplanation = 230,
 }
 
 impl StockMessage {
@@ -1335,6 +1338,11 @@ pub(crate) async fn proxy_description(context: &Context) -> String {
     translated(context, StockMessage::ProxyEnabledDescription).await
 }
 
+/// Stock string: `Messages in this chat use classic email and are not encrypted.`.
+pub(crate) async fn chat_unencrypted_explanation(context: &Context) -> String {
+    translated(context, StockMessage::ChatUnencryptedExplanation).await
+}
+
 impl Context {
     /// Set the stock string for the [StockMessage].
     ///