feat: support assetPrefix in next.config

## Summary

`assetPrefix` from `next.config` is currently not read or used anywhere in vinext. Setting it is silently ignored.

## Current behavior

In production, all static assets (fonts, JS chunks, etc.) are always served from:

```
{basePath}/assets/[file]-[hash].[ext]
```

The URL is generated by Vite's asset pipeline with no way to inject a CDN prefix.

## Expected behavior

When `assetPrefix` is set in `next.config`:

```ts
// next.config.ts
const nextConfig: NextConfig = {
assetPrefix: 'https://assets.example.com',
};
```

All asset URLs should be prefixed accordingly:

```
https://assets.example.com/assets/Font-[hash].woff2
https://assets.example.com/_next/static/chunks/...
```

## Why this matters

When deploying to Cloudflare Workers with a gateway pattern (a hub Worker routing requests to multiple service Workers), every request — including static assets like fonts and images — goes through the gateway Worker and incurs an invocation.

With `assetPrefix` support, static assets could be served from a dedicated subdomain (e.g. `assets-app.example.com`) backed by a Cloudflare Workers Assets-only deployment with `run_worker_first: false`. This would serve fonts, CSS, and JS bundles **directly from Cloudflare's edge with zero Worker invocations**.

## Investigation

Verified by reading `dist/config/next-config.js` — `resolveNextConfig` never extracts `assetPrefix`. The `vinext:local-fonts` Vite plugin (index.js lines 3153–3219) rewrites font paths to Vite static asset imports, but Vite's `base` (driven by `basePath`) is the only configurable prefix — there is no `assetPrefix` injection at any stage.

## Suggested approach

In `resolveNextConfig`, extract `assetPrefix` and pass it through to the Vite plugin. The plugin would then configure Vite's [`experimental.renderBuiltUrl`](https://vitejs.dev/guide/build#advanced-base-options) to prefix resolved asset URLs:

```ts
// vite.config equivalent
build: {
rollupOptions: {},
},
experimental: {
renderBuiltUrl(filename) {
return assetPrefix + '/' + filename;
}
}
```

This would cover JS/CSS chunks, font files, and images processed through Vite's asset pipeline.