fix: band-level windowed refine_layout + programmatic map_fields to prevent 91.5% content loss

Root cause: LLM receiving full 34k-char JRXML would regenerate from scratch
instead of modifying coordinates in-place, shrinking output to ~3k chars.

Solution (programmatic node control, not prompt engineering):

- New agent/jrxml_windower.py: decompose JRXML into header (never sent to
  LLM) + individual bands. Split bands >4000 chars at element boundaries.
  Reassemble with element count validation (>10% change = rollback).

- Rewrite refine_layout: per-band windowed LLM processing (~2-4k chars
  each). LLM cannot "reimagine" the entire report.

- Rewrite map_fields: 100% programmatic regex $F{field_N} -> real name
  replacement. Zero LLM calls, zero content loss.

- _sanitize_field_name: non-ASCII chars escaped to _uXXXX_ format for
  valid JRXML identifiers.

- Tests: 48 new unit tests (windower 28 + map_fields 20). All passing.
  Full suite 385 tests, zero regressions.

frontend/README.md

@@ -1,5 +1,52 @@
-# Vue 3 + TypeScript + Vite
+# JRXML Agent 前端
 
-This template should help get you started developing with Vue 3 and TypeScript in Vite. The template uses Vue 3 `<script setup>` SFCs, check out the [script setup docs](https://v3.vuejs.org/api/sfc-script-setup.html#sfc-script-setup) to learn more.
+Vue 3 + TypeScript + Vite + Pinia — JRXML 报表生成代理的 Web UI。
 
-Learn more about the recommended Project Setup and IDE Support in the [Vue Docs TypeScript Guide](https://vuejs.org/guide/typescript/overview.html#project-setup).
+## 技术栈
+
+- **Vue 3** (Composition API + `<script setup>`)
+- **TypeScript** 6.x
+- **Vite** 8.x
+- **Pinia** 3.x (状态管理)
+- **SSE** (Server-Sent Events) 流式响应
+
+## 组件结构
+
+```
+src/
+├── api/client.ts            SSE 客户端 + fetch 封装
+├── stores/
+│   ├── chat.ts              Pinia: 消息/流式/节点进度/文件
+│   ├── session.ts           Pinia: 会话 CRUD
+│   └── kb.ts                Pinia: 多租户知识库管理
+├── components/
+│   ├── Sidebar.vue          会话列表 + 下载 + 历史版本
+│   ├── ChatMessages.vue     消息列表渲染
+│   ├── ProcessSection.vue   处理过程折叠区
+│   ├── UnifiedInput.vue     统一输入框（文本+文件拖拽/粘贴）
+│   ├── SummaryCard.vue      结果摘要卡片
+│   ├── KbSelector.vue       KB 下拉选择器
+│   └── KbManager.vue        KB 管理面板（创建/上传/构建/删除）
+└── utils/format.ts          工具函数
+```
+
+## 开发
+
+```bash
+npm install
+npm run dev        # 启动开发服务器 (localhost:5173)
+npm run build      # 生产构建
+npx playwright test # E2E 测试
+```
+
+## SSE 事件流
+
+前端通过 `api.chat()` 发起 POST 请求，后端返回 `text/event-stream`：
+
+| 事件 | 说明 |
+|------|------|
+| `node_start` | 节点开始执行（含 node/label/step_index） |
+| `node_complete` | 节点执行完成（含 detail） |
+| `stream_token` | LLM 逐字输出 |
+| `agent_complete` | 全图执行完成（含 intent/status/jrxml_length/error 等） |
+| `agent_error` | 执行异常 |