fix: sync build artifacts to GitHub instead of empty source archives

Move GitHub release sync into build-app.yml as a final sync-to-github
job that runs after all 3 platform builds complete. This eliminates the
race condition where sync-release.yml triggered before artifacts were
uploaded to Gitea. The old sync-release.yml is changed to manual-only.

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

.gitea/workflows/sync-release.yml

@@ -1,8 +1,7 @@
 name: Sync Release to GitHub
 
 on:
-  release:
-    types: [published]
+  workflow_dispatch:
 
 jobs:
   sync-release: