feat: App - Support setting base URL during build

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
```

packages/app/src/app.tsx

@@ -284,6 +284,7 @@ export function AppInterface(props: {
               <Dynamic
                 component={props.router ?? Router}
                 root={(routerProps) => <RouterRoot appChildren={props.children}>{routerProps.children}</RouterRoot>}
+                base={import.meta.env.BASE_URL.replace(/\/$/, "") || undefined}
               >
                 <Route path="/" component={HomeRoute} />
                 <Route path="/:dir" component={DirectoryLayout}>

packages/app/src/entry.tsx

@@ -99,9 +99,11 @@ if (!(root instanceof HTMLElement) && import.meta.env.DEV) {
 
 const getCurrentUrl = () => {
   if (location.hostname.includes("opencode.ai")) return "http://localhost:4096"
+  const serverBaseUrl =
+    "/" + (import.meta.env.VITE_OPENCODE_SERVER_BASE_URL ?? "/").replace(/^\//, "").replace(/\/$/, "")
   if (import.meta.env.DEV)
-    return `http://${import.meta.env.VITE_OPENCODE_SERVER_HOST ?? "localhost"}:${import.meta.env.VITE_OPENCODE_SERVER_PORT ?? "4096"}`
-  return location.origin
+    return `http://${import.meta.env.VITE_OPENCODE_SERVER_HOST ?? "localhost"}:${import.meta.env.VITE_OPENCODE_SERVER_PORT ?? "4096"}{serverBaseUrl}`
+  return location.origin + serverBaseUrl
 }
 
 const getDefaultUrl = () => {

packages/app/src/env.d.ts

@@ -1,6 +1,7 @@
 import "solid-js"
 
 interface ImportMetaEnv {
+  readonly BASE_URL: string
   readonly VITE_OPENCODE_SERVER_HOST: string
   readonly VITE_OPENCODE_SERVER_PORT: string
 }

packages/app/vite.config.ts

@@ -2,6 +2,7 @@ import { defineConfig } from "vite"
 import desktopPlugin from "./vite"
 
 export default defineConfig({
+  base: process.env.VITE_BASE_URL || "/",
   plugins: [desktopPlugin] as any,
   server: {
     host: "0.0.0.0",