fix: support async browser scripts in build_eval_js

Wrap eval script in async IIFE and await Promise-like results.
Fixes Promise serialization returning '{}' for async skill scripts.

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src/browser/callback_backend.rs

@@ -436,7 +436,9 @@ fn build_eval_js(source_url: &str, script: &str) -> String {
     let events_url = escape_js_single_quoted(&events_endpoint_url(source_url));
 
     format!(
-        "(function(){{try{{var v=(function(){{return {script}}})();\
+        "(async function(){{try{{\
+         var v=await (async function(){{return {script}}})();\
+         if(v&&typeof v.then==='function'){{v=await v;}}\
          var t=(typeof v==='string')?v:JSON.stringify(v);\
          try{{callBackJsToCpp('{escaped_source_url}@_@'+window.location.href+'@_@{callback}@_@sgBrowserExcuteJsCodeByDomain@_@'+(t??''))}}catch(_){{}}\
          var j=JSON.stringify({{type:'callback',callback:'{callback}',request_url:'{escaped_source_url}',payload:{{value:(t??'')}}}});\