fix: simplify bake file to avoid locals/function blocks (buildx compat)

The Gitea runner's docker buildx doesn't support HCL locals{} or function{}
blocks (added in buildx 0.12+). Replace with plain variables: VERSION and
MAJOR_MINOR are pre-computed in a CI step and passed as env vars to bake.

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

.gitea/workflows/release.yaml

@@ -166,6 +166,14 @@ jobs:
           username: ${{ secrets.DOCKER_USER }}
           password: ${{ secrets.DOCKER_TOKEN }}
 
+      - name: Compute version tags
+        id: ver
+        run: |
+          V="${{ gitea.ref_name }}"
+          VER="${V#v}"
+          echo "version=$VER" >> "$GITHUB_OUTPUT"
+          echo "major_minor=$(echo "$VER" | cut -d. -f1-2)" >> "$GITHUB_OUTPUT"
+
       - name: Download ui build artifacts
         uses: actions/download-artifact@v3
         with: