ci: add GitHub Actions pipeline with tests, lint, and Pages deploy

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+# Least privilege by default; the deploy job widens this for itself.
+permissions:
+  contents: read
+
+# A newer run for the same ref cancels older in-flight ones.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Test & build (Node ${{ matrix.node }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        node: [20, 22]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node }}
+          cache: npm
+      - run: npm ci
+      - name: Typecheck
+        run: npx tsc --noEmit
+      - name: Lint
+        run: npm run lint
+      - name: Test
+        run: npm test
+      - name: Build
+        run: npm run build
+
+  # Build the production bundle once and upload it as a Pages artifact.
+  build-pages:
+    name: Build Pages artifact
+    needs: test
+    if: github.ref == 'refs/heads/master'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm run build
+      - uses: actions/configure-pages@v5
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: dist
+
+  deploy:
+    name: Deploy to GitHub Pages
+    needs: build-pages
+    if: github.ref == 'refs/heads/master'
+    runs-on: ubuntu-latest
+    permissions:
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - id: deployment
+        uses: actions/deploy-pages@v4

eslint.config.js

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+import js from '@eslint/js'
+import globals from 'globals'
+import reactHooks from 'eslint-plugin-react-hooks'
+import reactRefresh from 'eslint-plugin-react-refresh'
+import tseslint from 'typescript-eslint'
+
+// Flat ESLint config for the Vite + TS + React app. Lints src/ (and the few
+// root config files); the build output in dist/ is ignored.
+export default tseslint.config(
+  { ignores: ['dist'] },
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [js.configs.recommended, ...tseslint.configs.recommended],
+    languageOptions: {
+      ecmaVersion: 2020,
+      globals: globals.browser,
+    },
+    plugins: {
+      'react-hooks': reactHooks,
+      'react-refresh': reactRefresh,
+    },
+    rules: {
+      ...reactHooks.configs['recommended-latest'].rules,
+      'react-refresh/only-export-components': [
+        'warn',
+        { allowConstantExport: true },
+      ],
+    },
+  },
+)