This is needed to host OpenCode app under under some URL prefix.
This minimal change makes it possible to build OpenCode Web App with VITE_BASE_URL="/subdir/" and have it run under `http://<host>:<port>/subdir/` instead of root (`http://<host>:<port>/`).
Set `VITE_BASE_URL` env var during build. This automatically sets the `base` in Vite config. Vite generates correct URLs in `index.html`.
`import.meta.env.BASE_URL` is also used in routers. See https://vite.dev/config/shared-options#base
You can also set `VITE_OPENCODE_SERVER_BASE_URL` to specify the base URL prefix for the server.
Usage:
```
% VITE_BASE_URL="/XXX" VITE_OPENCODE_SERVER_BASE_URL="/" bun dev
$ vite
(!) "base" option should start with a slash.
VITE v7.1.4 ready in 320 ms
➜ Local: http://localhost:3000/XXX
```
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