fix(dev): proxy content assets to cli server

The CLI content server serves the content directory at the URL root, so
images and other content assets are requested as root-relative paths
(e.g. /adventures/map.png). The dev proxy only forwarded /content, so
those requests fell through to historyApiFallback and the browser got
index.html instead of the asset.

Forward any request that maps to a real file under content/ to the CLI
server, keeping /static/ bundles on the web dev server.

src/data-loader/file-index.ts

@@ -8,7 +8,8 @@
  * Dev 工作流：运行 CLI 服务器并在 rsbuild 配置中设置代理
  *   > npm run tttk serve ./content
  *   > npm run dev
- *  rsbuild 会将 /__CONTENT_INDEX.json、/__COMPLETIONS.json、/__CONTENT_REGISTRY.json、/content/ 代理到 CLI 服务器
+ *  rsbuild 会将 /__CONTENT_INDEX.json、/__COMPLETIONS.json、/__CONTENT_REGISTRY.json、
+ *  /.ttrpg 以及内容目录中的静态资源代理到 CLI 服务器
  */
 
 import {