ci: add release workflow with template lint and artifact packaging

- Validate all templates on push to main and tag v*
- On tag: package themes/ as zip + tar.gz and create GitHub release

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

.github/workflows/release.yml

@@ -0,0 +1,47 @@
+name: Release
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+
+jobs:
+  validate:
+    name: Validate Templates
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.21'
+
+      - name: Lint — render all templates
+        working-directory: tools
+        run: go run . preview all
+
+  package:
+    name: Package & Release
+    if: startsWith(github.ref, 'refs/tags/v')
+    needs: validate
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Package themes/
+        run: |
+          VERSION=${GITHUB_REF#refs/tags/}
+          ARCHIVE="gitea-mail-templates-${VERSION}"
+          mkdir -p dist
+          cp -r themes LICENSE README.md "dist/${ARCHIVE}"
+          cd dist
+          zip -r "${ARCHIVE}.zip" "${ARCHIVE}"
+          tar -czf "${ARCHIVE}.tar.gz" "${ARCHIVE}"
+
+      - name: Create Release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: dist/*.zip,dist/*.tar.gz
+          generate_release_notes: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}