Add build CI and automate fontdue-js bumps

.github/dependabot.yml

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+version: 2
+
+updates:
+  - package-ecosystem: npm
+    directory: "/"
+    schedule:
+      interval: daily
+    # Only fontdue-js. Without this allow-list Dependabot would also start
+    # opening PRs for React Router, React and every other dependency in the tree.
+    allow:
+      - dependency-name: fontdue-js
+    # Bump the pin in place (3.0.5 -> 3.0.6) rather than widening it to a range.
+    versioning-strategy: increase
+    open-pull-requests-limit: 1

.github/workflows/build.yml

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+name: build
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    env:
+      # Public staging backend; nothing secret here.
+      VITE_FONTDUE_URL: https://example.fontdue.xyz
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm run build
+
+  # Dependabot opens fontdue-js bumps (see .github/dependabot.yml). `needs: build`
+  # is what makes merging them unattended safe -- without it this would merge a
+  # release that doesn't compile against this framework.
+  automerge:
+    needs: build
+    if: github.event_name == 'pull_request' && github.event.pull_request.user.login == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - uses: dependabot/fetch-metadata@v2
+        id: meta
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      # Major bumps land in a human's inbox: those are the ones that break templates.
+      - if: steps.meta.outputs.update-type != 'version-update:semver-major'
+        run: gh pr merge --squash --delete-branch "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}