ci: bring back GitHub Pages auto-deploy

.github/workflows/deploy-github-pages.yml runs `npm run build:gh` on push
to main and uploads build/ via actions/upload-pages-artifact@v3 +
actions/deploy-pages@v4.

The build:gh script sets BUILD_PROFILE=gh, which switches svelte.config.js
basePath to /loto so assets resolve under tiennm99.github.io/loto.

Cloudflare Pages keeps deploying in parallel via the CF dashboard
(npm run build, root basePath, loto.miti99.com). Two URLs, no conflict.

web/.github/workflows/deploy-github-pages.yml

@@ -0,0 +1,41 @@
+name: Deploy to GitHub Pages
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: github-pages
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run build:gh
+      - uses: actions/configure-pages@v5
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: build
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - id: deployment
+        uses: actions/deploy-pages@v4