fix(web): add fallback for clipboard API in insecure contexts (#34092)

- Clipboard API requires secure context (HTTPS) to work
- Windows HTTP environments fail silently
- Add textarea element fallback for non-secure contexts

ui/src/ui/chat/copy-as-markdown.ts

@@ -17,11 +17,28 @@ async function copyTextToClipboard(text: string): Promise<boolean> {
     return false;
   }
 
+  // Try Clipboard API first (works in secure contexts/HTTPS)
   try {
     await navigator.clipboard.writeText(text);
     return true;
   } catch {
-    return false;
+    // Fallback for non-secure contexts (HTTP on Windows/localhost)
+    try {
+      // Use textarea element fallback for insecure contexts
+      const textarea = document.createElement("textarea");
+      textarea.value = text;
+      textarea.style.position = "fixed";
+      textarea.style.left = "-9999px";
+      textarea.style.top = "-9999px";
+      document.body.appendChild(textarea);
+      textarea.focus();
+      textarea.select();
+      const success = document.execCommand("copy");
+      document.body.removeChild(textarea);
+      return success;
+    } catch {
+      return false;
+    }
   }
 }