Add workflow: auto-update themes/index.json on theme changes

.github/workflows/update-themes-index.yml

@@ -0,0 +1,46 @@
+name: Update themes index
+
+on:
+  push:
+    branches: [gh-pages]
+    paths:
+      - 'themes/**'
+      - '!themes/index.json'
+
+jobs:
+  update-index:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: gh-pages
+
+      - name: Regenerate themes/index.json
+        run: |
+          python3 - << 'EOF'
+          import json, os
+          themes_root = 'themes'
+          result = {}
+          for cat in sorted(os.listdir(themes_root)):
+              cat_path = os.path.join(themes_root, cat)
+              if not os.path.isdir(cat_path):
+                  continue
+              files = sorted(f for f in os.listdir(cat_path) if f.endswith('.yaml'))
+              themes = [{'slug': cat + '/' + f, 'label': f[:-5].replace('-', ' ')} for f in files]
+              if themes:
+                  result[cat] = themes
+          with open('themes/index.json', 'w') as fh:
+              json.dump(result, fh, indent=2)
+          total = sum(len(v) for v in result.values())
+          print(f'Updated: {len(result)} categories, {total} themes')
+          EOF
+
+      - name: Commit and push
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git add themes/index.json
+          git diff --cached --quiet || git commit -m "Auto-update themes/index.json"
+          git push