feat(ui): canvas-first redesign with a self-hosted draw.io

The canvas now fills the window and the chat floats on its right. draw.io is
served from our own origin (public/drawio, downloaded at build time and
checked against the release's SHA-256), so the app can drive the editor
directly:

- every AI change becomes a version card with a thumbnail; compare, restore
  or undo it, and Ctrl+Z on the canvas takes an AI change back in one step
- shapes the AI just changed are highlighted on the canvas
- selected shapes appear as a chip and are sent to the model
- the app's own canvas toolbar, shape library and page tabs replace
  draw.io's chrome
- new start screen, session rename and switcher, open .drawio files
- settings split into Models & keys, Appearance, Advanced and About
- phones switch between canvas and chat
- dark mode keeps the diagram on a white sheet and no longer flashes

With an external draw.io (NEXT_PUBLIC_DRAWIO_BASE_URL) the app falls back to
draw.io's own toolbar and the features above that need same origin are off.

The chat logic moved from the 1500-line chat panel into a chat engine plus
small UI components; zustand holds canvas, versions, settings and UI state.

docs/cn/FAQ.md

@@ -18,9 +18,11 @@
 
 **问题**: 内网环境提示"找不到 embed.diagrams.net 的服务器 IP 地址"
 
-**关键点**: `NEXT_PUBLIC_*` 环境变量是**构建时**变量，会被打包到 JS 代码中，**运行时设置无效**！
+**关键点**: 应用现在自带一份 draw.io，从 `/drawio` 提供，浏览器不再需要访问 `embed.diagrams.net`。如果仍然看到这个错误，说明镜像是用 `NEXT_PUBLIC_DRAWIO_BASE_URL=https://embed.diagrams.net`（旧版 Dockerfile 的默认值）构建的，去掉这个构建参数重新构建即可。详见[离线部署](./offline-deployment.md)。
 
-**解决方案**: 必须在构建时通过 `args` 传入：
+`NEXT_PUBLIC_*` 环境变量是**构建时**变量，会被打包到 JS 代码中，**运行时设置无效**！
+
+**如果仍想使用单独的 draw.io 服务器**，在构建时通过 `args` 传入：
 
 ```yaml
 # docker-compose.yml