feat(ai): V2 深度增强 — SSE 流式/全局助手/内容安全/多角色覆盖

对标 Khanmigo/Duolingo Max/Squirrel AI/Century Tech 实现：

- SSE 流式响应：createAiChatCompletionStream AsyncGenerator + /api/ai/chat/stream SSE 端点 + useAiChatStream hook（AbortController 停止生成 + localStorage 持久化）

- Markdown 渲染：AiMarkdownRenderer（react-markdown + remark-gfm + 代码块/表格/列表 + hover 复制按钮）

- 全局 AI 助手：AiAssistantWidget 浮动按钮 + Sheet 侧抽屉 + usePathname 路由推断上下文（7 类场景系统提示）+ dashboard layout 全局注入 AiClientProvider

- 内容安全：content-safety.ts 多层过滤（输入/输出安全过滤 + 每日限制 student 50/teacher 200/parent 30/admin 500 + 学生苏格拉底模式），COPPA/FERPA K12 合规

- 多角色 AI 覆盖：家长端 AiChildSummary（学情摘要）+ 管理员端 AiUsageDashboard（使用监控）+ 学生端 AiStudyPath（个性化学习路径）

- i18n 修复：8 处错误键引用 + zh-CN/en ai.json 全面扩展

- 架构文档 004/005 同步更新

src/app/(dashboard)/layout.tsx

@@ -1,6 +1,34 @@
 import { AppSidebar } from "@/modules/layout/components/app-sidebar"
 import { SidebarProvider } from "@/modules/layout/components/sidebar-provider"
 import { SiteHeader } from "@/modules/layout/components/site-header"
+import {
+  AiClientProvider,
+} from "@/modules/ai/context/ai-client-provider"
+import { AiAssistantWidget } from "@/modules/ai/components/ai-assistant-widget"
+import {
+  aiChatAction,
+  suggestSimilarQuestionsAction,
+  suggestGradingAction,
+  generateLessonContentAction,
+  generateQuestionVariantAction,
+  analyzeWeaknessAction,
+  generateChildSummaryAction,
+  recommendStudyPathAction,
+  getAiUsageStatsAction,
+} from "@/modules/ai/actions"
+import type { AiClientService } from "@/modules/ai/types"
+
+const aiClientService: AiClientService = {
+  chat: aiChatAction,
+  suggestSimilarQuestions: suggestSimilarQuestionsAction,
+  suggestGrading: suggestGradingAction,
+  generateLessonContent: generateLessonContentAction,
+  generateQuestionVariant: generateQuestionVariantAction,
+  analyzeWeakness: analyzeWeaknessAction,
+  generateChildSummary: generateChildSummaryAction,
+  recommendStudyPath: recommendStudyPathAction,
+  getAiUsageStats: getAiUsageStatsAction,
+}
 
 export default function DashboardLayout({
   children,
@@ -8,14 +36,17 @@ export default function DashboardLayout({
   children: React.ReactNode
 }) {
   return (
-    <SidebarProvider sidebar={<AppSidebar />}>
-      <a href="#main-content" className="sr-only focus:not-sr-only focus:absolute focus:z-50 focus:p-4 focus:bg-background focus:text-foreground focus:border focus:border-border focus:rounded-md focus:m-2">
-        Skip to main content
-      </a>
-      <SiteHeader />
-      <main id="main-content" className="flex-1 overflow-auto p-6">
-        {children}
-      </main>
-    </SidebarProvider>
+    <AiClientProvider service={aiClientService}>
+      <SidebarProvider sidebar={<AppSidebar />}>
+        <a href="#main-content" className="sr-only focus:not-sr-only focus:absolute focus:z-50 focus:p-4 focus:bg-background focus:text-foreground focus:border focus:border-border focus:rounded-md focus:m-2">
+          Skip to main content
+        </a>
+        <SiteHeader />
+        <main id="main-content" className="flex-1 overflow-auto p-6">
+          {children}
+        </main>
+        <AiAssistantWidget />
+      </SidebarProvider>
+    </AiClientProvider>
   )
 }