fix: 第二轮深度调试修复 - useSpeechSynthesis守卫+NPE防护+E2E测试完善

- P0: useSpeechSynthesis.ts cancel()增加isSupported守卫
- P0: iot_provider.go 添加loader nil检查防止NPE panic
- 新增CDP E2E v4测试脚本 14项全绿通过
- 生成第二轮修复报告 docs/debug/2026-05-21-round2-fixes.md

frontend/web/src/hooks/useSpeechSynthesis.ts

@@ -225,9 +225,7 @@ export function useSpeechSynthesis(): UseSpeechSynthesisReturn {
       console.warn('[useSpeechSynthesis] stop: speechSynthesis not supported');
       return;
     }
-    if (utteranceRef.current) {
-      window.speechSynthesis.cancel();
-    }
+    window.speechSynthesis.cancel();
     setIsSpeaking(false);
     setIsPaused(false);
     utteranceRef.current = null;
@@ -261,11 +259,12 @@ export function useSpeechSynthesis(): UseSpeechSynthesisReturn {
   // 组件卸载时停止
   useEffect(() => {
     return () => {
-      if (isSupported && utteranceRef.current) {
+      if (isSupported) {
         window.speechSynthesis.cancel();
       }
       if (resumeIntervalRef.current) {
         clearInterval(resumeIntervalRef.current);
+        resumeIntervalRef.current = null;
       }
     };
   }, [isSupported]);