Revert "render system-user-interruption as styled yellow box in prose"

This reverts commit 4e455e2dd3.

gui/chatblockmodel.cpp

@@ -194,28 +194,26 @@ QString ChatBlockModel::highlightCode(const QString &code, const QString &langua
 QString ChatBlockModel::renderMarkdown(const QString &markdown) const
 {
     // Pre-process: render <system-user-interruption> blocks as styled boxes.
-    // These appear as prose between code fences (the server closes the tool
-    // output fence before the interruption and reopens it after).
+    // Manual global-match loop instead of replace-with-lambda, which requires
+    // Qt 6.5+ and isn't available on all target systems.
     static const QRegularExpression interruptionRe(
         R"(<system-user-interruption>\n([\s\S]*?)\n</system-user-interruption>)");
-    QString processed = markdown;
+    QString processed;
     {
         int cursor = 0;
-        QString rebuilt;
         auto it = interruptionRe.globalMatch(markdown);
         while (it.hasNext()) {
             const auto match = it.next();
-            rebuilt += markdown.mid(cursor, match.capturedStart() - cursor);
+            processed += markdown.mid(cursor, match.capturedStart() - cursor);
             const QString content = match.captured(1).toHtmlEscaped();
-            rebuilt += QStringLiteral(
+            processed += QStringLiteral(
                 "<div style=\"background:#fff8e1;border:1px solid #ffca28;"
                 "border-radius:4px;padding:8px;margin:8px 0;white-space:pre-wrap;\">"
                 "\u26A0\uFE0F <strong>User interruption</strong>\n")
                 + content + QStringLiteral("</div>");
             cursor = match.capturedEnd();
         }
-        rebuilt += markdown.mid(cursor);
-        processed = rebuilt;
+        processed += markdown.mid(cursor);
     }
 
     static const QRegularExpression fenceRe(