Remove v* tag trigger from flutter-build to fix double-runs

publish.yml already calls flutter-build via workflow_call on v* tags,
so the direct push trigger was causing duplicate workflow runs.

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

.github/workflows/flutter-build.yml

@@ -4,8 +4,6 @@ on:
   push:
     branches:
       - main
-    tags:
-      - 'v*'
     paths:
       - 'mobile/lib/**'
       - 'mobile/android/**'