feat(ssa): Implement dual-channel architecture Phase 1-3 (QPER + LLM Agent pipeline)

Completed:
- Phase 1: DB schema (execution_mode + ssa_agent_executions), ModeToggle component, Session PATCH API
- Phase 2: AgentPlannerService + AgentCoderService (streaming) + CodeRunnerService + R Docker /execute-code endpoint
- Phase 3: AgentCodePanel (3-step confirmation UI), SSE event handling (7 agent events), streaming code display
- Three-step confirmation pipeline: plan -> user confirm -> stream code -> user confirm -> execute R code -> results
- R Docker sandbox /execute-code endpoint with 120s timeout + block_helpers preloaded
- E2E dual-channel test script (8 tests)
- Updated R engine architecture doc (v1.5) and SSA module status doc (v4.0)

Technical details:
- AgentCoderService uses LLM streaming (chatStream) for real-time code generation feedback
- ReviewerAgent temporarily disabled, prioritizing Plan -> Code -> Execute flow
- CodeRunnerService wraps user code with auto data loading (df variable injection)
- Frontend handles agent_planning, agent_plan_ready, code_generating, code_generated, code_executing, code_result events
- ask_user mechanism used for plan and code confirmation steps

Files: 24 files (4 new services, 2 new components, 1 migration, 1 E2E test, 16 modified)
Made-with: Cursor

frontend-v2/src/modules/ssa/components/SSAChatPane.tsx

@@ -43,6 +43,7 @@ import { ClarificationCard } from './ClarificationCard';
 import { AskUserCard } from './AskUserCard';
 import type { AskUserResponseData } from './AskUserCard';
 import { ThinkingBlock } from '@/shared/components/Chat';
+import { ModeToggle } from './ModeToggle';
 import type { ClarificationCardData, IntentResult } from '../types';
 
 export const SSAChatPane: React.FC = () => {
@@ -285,14 +286,17 @@ export const SSAChatPane: React.FC = () => {
             {currentSession?.title || '新的统计分析'}
           </span>
         </div>
-        <EngineStatus
-          isExecuting={isExecuting}
-          isLoading={isLoading || isPlanLoading}
-          isUploading={uploadStatus === 'uploading' || uploadStatus === 'parsing'}
-          isProfileLoading={isProfileLoading || dataProfileLoading}
-          isStreaming={isGenerating}
-          streamIntent={currentIntent}
-        />
+        <div className="chat-header-right">
+          {currentSession && <ModeToggle />}
+          <EngineStatus
+            isExecuting={isExecuting}
+            isLoading={isLoading || isPlanLoading}
+            isUploading={uploadStatus === 'uploading' || uploadStatus === 'parsing'}
+            isProfileLoading={isProfileLoading || dataProfileLoading}
+            isStreaming={isGenerating}
+            streamIntent={currentIntent}
+          />
+        </div>
       </header>
 
       {/* Chat Messages */}