ci: add GitHub Actions CI workflow + README badges

- CI runs build + type check on Node 18/20/22 for push & PRs
- Add CI status, license, and Node.js version badges to README
- Replace placeholder screenshot with contribution note

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [18, 20, 22]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Type check
+        run: npx tsc --noEmit
+
+      - name: Build
+        run: npm run build
+
+      - name: Check bundle size
+        run: |
+          echo "## Bundle sizes" >> $GITHUB_STEP_SUMMARY
+          echo '```' >> $GITHUB_STEP_SUMMARY
+          du -sh dist/ >> $GITHUB_STEP_SUMMARY
+          find dist/assets -name '*.js' -exec du -sh {} \; | sort -rh >> $GITHUB_STEP_SUMMARY
+          echo '```' >> $GITHUB_STEP_SUMMARY