fix(ci): remove dart pub audit step (unsupported in runner SDK)

The stable Flutter SDK on the CI runner does not include
the `dart pub audit` subcommand. Trivy scan still covers
dependency security.

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

.gitea/workflows/ci.yaml

@@ -96,9 +96,6 @@ jobs:
         run: dart pub outdated
         continue-on-error: true
 
-      - name: Security audit
-        run: dart pub audit
-
       - name: Trivy filesystem scan
         run: |
           set -e