feat: 前后端分离架构 — FastAPI SSE后端 + Vue 3前端

将单体 Streamlit 应用拆分为三层架构：
- api_server.py: FastAPI SSE 流式后端 (端口 8000)
- frontend/: Vue 3 + Vite + Pinia 聊天前端 (端口 5173)
- agent/graph.py: 新增 node_start 回调支持
- 更新启动脚本为三服务模式

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/tsconfig.app.json

@@ -0,0 +1,14 @@
+{
+  "extends": "@vue/tsconfig/tsconfig.dom.json",
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
+    "types": ["vite/client"],
+
+    /* Linting */
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "erasableSyntaxOnly": true,
+    "noFallthroughCasesInSwitch": true
+  },
+  "include": ["src/**/*.ts", "src/**/*.tsx", "src/**/*.vue"]
+}