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>

.gitea/workflows/build.yml

@@ -36,6 +36,13 @@ jobs:
           username: ${{ gitea.actor }}
           password: ${{ secrets.REGISTRY_TOKEN }}
 
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: shadowdao
+          password: ${{ secrets.GH_PAT }}
+
       - name: Build and push container image
         uses: docker/build-push-action@v5
         with:
@@ -46,5 +53,7 @@ jobs:
           tags: |
             ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
             ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ gitea.sha }}
+            ghcr.io/shadowdao/triple-c-sandbox:latest
+            ghcr.io/shadowdao/triple-c-sandbox:${{ gitea.sha }}
           cache-from: type=gha
           cache-to: type=gha,mode=max