fix(ssa): Fix Agent mode chat leaking plan/result details + navigation reset

Backend (ChatHandlerService):
- Replace LLM-generated hints with fixed-text hints for plan/code/result steps
  to prevent SystemPrompt intent instruction conflict causing verbose chat output
- Add sendFixedHint() helper for deterministic SSE text delivery (no LLM call)
- Cancel old plan_pending/code_pending executions when regenerating plan
- Eliminates 3 unnecessary LLM calls per analysis cycle (faster response)

Frontend (SSAModule):
- Use location.key as useEffect dependency to ensure store reset on every
  navigation (fixes stale session when re-entering from other modules)
- TopNavigation uses replace:true for active module re-click to avoid
  browser history clutter

Tested: Agent mode plan/code/result hints now show brief fixed text in chat,
detailed content exclusively in right workspace panel

Made-with: Cursor

frontend-v2/src/modules/ssa/index.tsx

@@ -7,6 +7,7 @@
  * - 右侧工作区（动态 60% 宽度）
  */
 import React, { useEffect } from 'react';
+import { useLocation } from 'react-router-dom';
 import { useSSAStore } from './stores/ssaStore';
 import SSAWorkspace from './SSAWorkspace';
 import './styles/ssa.css';
@@ -14,11 +15,13 @@ import './styles/ssa-workspace.css';
 
 const SSAModule: React.FC = () => {
   const { reset } = useSSAStore();
+  const location = useLocation();
 
-  // 组件挂载时重置 store，确保不同用户看到独立的状态
+  // location.key 在每次 navigate() 调用时都会变化（即使 URL 相同），
+  // 确保从其他模块返回或重复点击导航栏都能重置到全新状态
   useEffect(() => {
     reset();
-  }, [reset]);
+  }, [location.key, reset]);
 
   return <SSAWorkspace />;
 };