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

web/docs/deployment-guide.md

@@ -32,18 +32,26 @@ pushes to `main` trigger automatic builds + deploys.
 
 No GitHub Actions involved; no repo secrets needed.
 
-## Manual GH Pages Deploy (optional)
+## GitHub Pages (parallel auto-deploy)
 
-If you ever want to push a build to GitHub Pages by hand:
+`.github/workflows/deploy-github-pages.yml` runs on every push to `main`,
+builds with `npm run build:gh` (basePath `/loto`), and publishes `build/`
+as a Pages artifact.
+
+URL: `https://tiennm99.github.io/loto`.
+
+CF Pages and GH Pages run in parallel — different URLs, different
+artifacts, no conflict. Drop GH Pages by deleting the workflow file.
+
+### Manual GH Pages Deploy
+
+If you'd rather export by hand:
 
 ```bash
 npm run build:gh
 # upload build/ to the GH Pages target
 ```
 
-The repo no longer auto-deploys to GH Pages — there's no GitHub Actions
-workflow. CF Pages is the only auto-deploy target.
-
 ## Development Environment
 
 ### Local Dev
@@ -147,10 +155,11 @@ Generates:
 
 ## CI/CD Pipeline
 
-Cloudflare Pages handles CI/CD automatically:
-- GitHub Pages deployment removed (Cloudflare is primary).
-- Manual fallback: `npm run build:gh && git push` (if needed).
-- No GitHub Actions workflow for automatic GH Pages deploy.
+Both providers auto-deploy on push to `main`:
+- **Cloudflare Pages** — wired via the CF dashboard, builds with `npm run build`,
+  publishes to `loto.miti99.com`.
+- **GitHub Pages** — wired via `.github/workflows/deploy-github-pages.yml`,
+  builds with `npm run build:gh`, publishes to `tiennm99.github.io/loto`.
 
 ## Performance Checklist
 

web/docs/project-overview-pdr.md

@@ -31,7 +31,7 @@ State is entirely client-side. Each page/card instance uses a unique localStorag
 
 ## Deployment
 
-- **Production**: Cloudflare Pages auto-deploys from `main` branch (configured via the CF dashboard, no GitHub Actions). Custom domain `loto.miti99.com`. `npm run build:gh` is a manual fallback for GitHub Pages at the `/loto` basePath.
+- **Production**: dual auto-deploy on push to `main` — Cloudflare Pages at `loto.miti99.com` (CF dashboard, root basePath) and GitHub Pages at `tiennm99.github.io/loto` (`.github/workflows/deploy-github-pages.yml`, `/loto` basePath via `npm run build:gh`).
 - **Development**: `npm run dev` (local), `npm run dev:codeserver` (code-server via proxy).
 - **Build**: `npm run build` generates static export to `build/` directory.