fix(updater): fallback to GitHub API for release checking

[Veslydev]

Problem

The auto updater was failing with 404 (Not Found) because PackageManager.GetIndexAsync attempted to fetch https://{owner}.github.io/{name}/releases.json which does not exist for repositories that do not publish index files via GitHub Pages.

Solution

1. Added fallback in PackageManager.GetIndexAsync to query GitHub REST API (https://api.github.com/repos/{owner}/{name}/releases) when
   eleases.json returns HTTP 404.
2. Built dynamic PackageReleaseIndex and PackageManifest objects from GitHub release tags and assets.
3. Updated BuildInformation.VersionText to dynamically reference SiteLinkAPI.ApiVersionText to avoid desyncs.
4. Bumped version to 2.1.3.