Switch distribution from Gitea to GitHub/GHCR

Work VPN blocks repo.anhonesthost.net, breaking update checks and image
pulls. Move all user-facing distribution to GitHub (releases API) and
GHCR (container images) while keeping Gitea as the source of truth for
development and CI.

- CI: push container images to GHCR alongside Gitea registry
- App updates: switch releases API to api.github.com, filter by asset
  filename instead of tag suffix for unified releases
- Image updates: switch registry to ghcr.io with anonymous token auth
- Container pull: point REGISTRY_IMAGE to ghcr.io/shadowdao/triple-c-sandbox
- Rename GiteaRelease/GiteaAsset structs to GitHubRelease/GitHubAsset

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/src/models/update_info.rs

@@ -18,19 +18,19 @@ pub struct ReleaseAsset {
     pub size: u64,
 }
 
-/// Gitea API release response (internal).
+/// GitHub API release response (internal).
 #[derive(Debug, Clone, Deserialize)]
-pub struct GiteaRelease {
+pub struct GitHubRelease {
     pub tag_name: String,
     pub html_url: String,
     pub body: String,
-    pub assets: Vec<GiteaAsset>,
+    pub assets: Vec<GitHubAsset>,
     pub published_at: String,
 }
 
-/// Gitea API asset response (internal).
+/// GitHub API asset response (internal).
 #[derive(Debug, Clone, Deserialize)]
-pub struct GiteaAsset {
+pub struct GitHubAsset {
     pub name: String,
     pub browser_download_url: String,
     pub size: u64,