frontend/web: compile using relative base path instead of absolute

The default value for `base` is `/`, resulting in

```
<script type="module" crossorigin src="/assets/index-98aa38f3.js"></script>
<link rel="stylesheet" href="/assets/index-f26e81d9.css">
```

in `frontends/web/build/index.html`. For embedded use, the vite docs
recommend using a relative path:

```
<script type="module" crossorigin src="./assets/index-98aa38f3.js"></script>
<link rel="stylesheet" href="./assets/index-f26e81d9.css">
```

For Qt and Android, this does not seem to make a difference. On iOS
however, the way that local assets are looked up, it has to be
relative.

frontends/web/vite.config.mjs

@@ -5,6 +5,9 @@ import checker from 'vite-plugin-checker';
 
 export default defineConfig(() => {
   return {
+    // Relative base path so the js/css files are referenced with `./index-...js` instead of
+    // `/index-...js`. This makes it easier to find these files in iOS.
+    base: './',
     build: {
       modulePreload: false,
       outDir: 'build',