feat(preview): add useMediaPath(path) to resolve media URLs for Studio preview

[maximepvrt]

Is your feature request related to a problem? Please describe

In Content Studio Preview, media paths only work correctly when returned directly from Content composables.
If a media path is dynamically built (e.g. ${p.path}/thumbnail.jpg), the URL is not rewritten/proxied in Preview, so the image does not load, even though it works fine in production.

Describe the solution you'd like

Introduce a composable useMediaPath(path) that resolves a media path to the correct URL depending on the context:

• In Preview: return a proxied/rewritten URL so the asset loads.
• Outside Preview: returns the path param.

Example usage:

const { data: posts } = await useAsyncData(
  'posts',
  () => queryCollection('blog').order('date', 'DESC').all(),
  {
    transform: (posts) => posts.map((p) => {
      p.image = useMediaPath(`${p.path}/thumbnail.jpg`)
      p.to = p.path
      return p
    })
  }
)