Make preview build workflow manual-only

Trigger is workflow_dispatch exclusively so builds happen only when
explicitly requested from the Actions UI, not on every branch push.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-app-preview.yml

@@ -5,12 +5,6 @@ name: Build App (Preview)
 # smoke-testing of feature branches before they merge.
 
 on:
-  push:
-    branches-ignore: [main]
-    paths:
-      - "app/**"
-      - "VERSION"
-      - ".gitea/workflows/build-app-preview.yml"
   workflow_dispatch:
 
 jobs: