feat: show dev-<sha> version string in nav for dev builds

Production images continue to display the semver (v1.x.x). Dev images
built by CI now receive BUILD_VERSION=dev-<7-char-sha> via a Docker ARG,
and app.js skips the v prefix for non-semver strings.

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

.gitea/workflows/ci.yml

@@ -40,11 +40,15 @@ jobs:
           username: ${{ gitea.actor }}
           password: ${{ secrets.TOKEN }}
 
+      - name: Compute short SHA
+        run: echo "SHORT_SHA=${GITEA_SHA::7}" >> $GITEA_ENV
+
       - name: Build and push
         uses: docker/build-push-action@v5
         with:
           context: .
           push: true
+          build-args: BUILD_VERSION=dev-${{ env.SHORT_SHA }}
           tags: |
             ${{ env.IMAGE }}:dev
             ${{ env.IMAGE }}:dev-${{ gitea.sha }}