ci: Add deploy-demo workflow for GitHub Pages

.github/workflows/deploy-demo.yml

@@ -0,0 +1,73 @@
+name: Deploy Demo to GitHub Pages
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'client/**'
+      - '.github/workflows/deploy-demo.yml'
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  sync-and-deploy:
+    name: 🎭 Sync & Deploy Demo
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Checkout demo branch
+        uses: actions/checkout@v4
+        with:
+          ref: demo
+          fetch-depth: 0
+
+      - name: Sync main into demo
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git fetch origin main
+          # Merge main into demo, keeping demo's versions on conflict
+          git merge origin/main --no-edit -X ours || {
+            echo "⚠️ Merge conflict — resolving with demo branch versions"
+            git checkout --ours .
+            git add .
+            git commit --no-edit
+          }
+          git push origin demo
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '18'
+          cache: 'npm'
+          cache-dependency-path: client/package-lock.json
+
+      - name: Install dependencies
+        working-directory: client
+        run: npm ci --legacy-peer-deps
+
+      - name: Build demo
+        working-directory: client
+        env:
+          REACT_APP_DEMO_MODE: 'true'
+          PUBLIC_URL: '/RunWatch'
+        run: npm run build
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: client/build
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4