ci: add GitHub Actions workflow to deploy to GitHub Pages

- Add deploy.yml: test → build → deploy pipeline
- Update next.config.mjs with basePath/assetPrefix for /pikachu/ subpath
- Uses actions/deploy-pages for GitHub Pages deployment

.github/workflows/deploy.yml

@@ -0,0 +1,48 @@
+name: Deploy to GitHub Pages
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  test:
+    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 test
+
+  build-deploy:
+    needs: test
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run build-nolog
+      - uses: actions/configure-pages@v5
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: dist
+      - id: deployment
+        uses: actions/deploy-pages@v4