fix: use git rev-parse for short SHA in build-dev

$GITEA_SHA is unset on Gitea runners — the nav showed "dev-" with an
empty SHA. git rev-parse --short HEAD works regardless of runner env vars.

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

.gitea/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
           password: ${{ secrets.TOKEN }}
 
       - name: Compute short SHA
-        run: echo "SHORT_SHA=${GITEA_SHA::7}" >> $GITEA_ENV
+        run: echo "SHORT_SHA=$(git rev-parse --short HEAD)" >> $GITEA_ENV
 
       - name: Build and push
         uses: docker/build-push-action@v5