Function fetch_manifest 

pub async fn fetch_manifest(
    wasm_url: &OxideUrl,
) -> Result<Option<AppManifest>, String>

Fetches the manifest for a module URL (HTTP/HTTPS or file://).

Returns Ok(None) when the app has no manifest (missing file, HTTP 404, non-.wasm URL), Err with a human-readable message when a manifest exists but cannot be parsed.