fix: Store plaintext in mime_headers of truncated sent messages (#6273)

This fixes HTML display of truncated (long) sent messages ("Show full message" in UIs). Before,
incorrect HTML was stored (with missing line breaks etc.) for them. Now stored plaintext is
formatted to HTML upon calling `MsgId::get_html()` and this results in the same HTML as on a
receiver side.

src/chat.rs

@@ -2104,13 +2104,19 @@ impl Chat {
         } else {
             None
         };
+        let new_mime_headers = new_mime_headers.map(|s| new_html_mimepart(s).build().as_string());
         let new_mime_headers = new_mime_headers.or_else(|| match was_truncated {
-            true => Some(msg.text.clone()),
+            // We need to add some headers so that they are stripped before formatting HTML by
+            // `MsgId::get_html()`, not a part of the actual text. Let's add "Content-Type", it's
+            // anyway a useful metadata about the stored text.
+            true => Some(
+                "Content-Type: text/plain; charset=utf-8\r\n\r\n".to_string() + &msg.text + "\r\n",
+            ),
             false => None,
         });
         let new_mime_headers = match new_mime_headers {
             Some(h) => Some(tokio::task::block_in_place(move || {
-                buf_compress(new_html_mimepart(h).build().as_string().as_bytes())
+                buf_compress(h.as_bytes())
             })?),
             None => None,
         };