feat(plugins): serve plugin UI assets over the dbx-plugin scheme

The workbench sandbox inlines only the entry script, so code-split
chunks had no runtime address and dynamic imports 404'd. Serve each
installed plugin's ui.root through dbx-plugin://localhost/<id>/<path>
(WebView2 maps the scheme to http://dbx-plugin.localhost), inject the
platform-correct form as the sandbox document <base>, and allow it in
the resource CSP. Reads reuse the registry's ui-asset validation
(traversal confinement, 32 MiB cap, sha256 etag) and answer CORS with
ACAO:* for the opaque-origin iframe; responses are no-store so plugin
updates land immediately.

Existing plugins are unaffected. Builds that emit relative asset URLs
(vite base "./" / webpack publicPath "./") additionally regain lazy
loading and font loading; documented for plugin authors in both
languages.

docs/content/docs/plugin-development.cn.mdx

@@ -431,6 +431,10 @@ env 消息的 `theme` 结构为 `{ appearance, tokens, editor? }`。`appearance`
 
 UI 必须构建为包内静态资源，不要把 Vite 开发服务器或 CDN 的脚本地址写入发行包。宿主把入口及资源转换成沙箱可加载内容；CSP 报错时检查构建产物是否存在、路径是否落在 `ui.root` 内，并使用匹配版本的 CLI 复现。用插件自己的对话框组件处理删除/重命名，不要依赖沙箱中的原生 `alert`/`confirm`/`prompt`。
 
+#### 懒加载与代码分割
+
+宿主会把入口脚本内联进沙箱文档，`ui/` 下的其余资源在运行时通过 `dbx-plugin` 协议保持可寻址：`dbx-plugin://localhost/<插件id>/<ui.root内相对路径>`（WebView2 上映射为 `http://dbx-plugin.localhost/<插件id>/…`）。宿主会把平台对应的形式注入为沙箱文档的 `<base>`，并放行资源类 CSP，因此打包后的插件可以正常动态 `import()` 代码分割的 chunk——前提是构建产物使用**相对**资源地址：Vite 设 `base: "./"`、webpack 设 `publicPath: "./"`，让 chunk 和 CSS 资源地址基于注入的 base 解析而不是宿主 origin。绝对路径 `/assets/…` 携带不了插件 id，会 404。该协议只读文件、限定在每个插件的 `ui.root` 内，且插件更新后不会沿用旧缓存。
+
 ## 原生 Sidecar 和 RPC
 
 只有浏览器沙箱无法完成的能力才需要原生后端，例如 S3/SSH 协议、系统凭据、长连接或高性能计算。Sidecar 不是操作系统沙箱，会以当前用户权限运行。

docs/content/docs/plugin-development.mdx

@@ -431,6 +431,10 @@ Treat `terminal.fontSize`-style concerns as plugin-owned: the host never dictate
 
 Build the UI as packaged static assets, not references to a Vite server or CDN scripts. The host converts the entry and assets into sandbox-loadable content. For CSP errors, verify build outputs and confinement under `ui.root`, and reproduce with the matching CLI version. Use plugin-owned dialogs for delete/rename rather than relying on native `alert`/`confirm`/`prompt` inside the sandbox.
 
+#### Lazy loading and code splitting
+
+The host inlines the entry script into the sandbox document; further `ui/` assets stay addressable at runtime through the `dbx-plugin` scheme — `dbx-plugin://localhost/<plugin-id>/<path-under-ui.root>` (WebView2 maps it to `http://dbx-plugin.localhost/<plugin-id>/…`). The host injects the platform-correct form as the sandbox document's `<base>` and allows it in the resource CSP, so dynamic `import()` of code-split chunks works in packaged plugins — provided the build emits **relative** asset URLs: set `base: "./"` (Vite) or `publicPath: "./"` (webpack) so chunk and CSS asset URLs resolve against the injected base instead of the host origin. Absolute `/assets/…` references cannot carry the plugin id and will 404. The scheme serves files only, confined to each plugin's `ui.root`, and responses are never cached across plugin updates.
+
 ## Native Sidecars and RPC
 
 Use a native backend only when the browser sandbox cannot provide the capability, such as S3/SSH protocols, system credentials, long-lived connections, or CPU-heavy work. A Sidecar is not an OS sandbox and runs with the current user's privileges.