Add GitHub Pages deployment workflow for the demo site

Builds core -> widget -> demo and publishes packages/demo/dist via
actions/upload-pages-artifact + deploy-pages. Also makes the manifest and
service worker's precache list path-relative (instead of root-absolute)
so the same build works whether it's served from a domain root or a
GitHub Pages project-site subpath like /bitlogin/.

Repo still needs Settings -> Pages -> Source set to "GitHub Actions" once
if it has never been enabled before.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GWJbXXhvTx9wo59jSz57J6

.github/workflows/pages.yml

@@ -0,0 +1,47 @@
+name: Deploy demo 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:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+
+      - run: npm ci
+
+      - run: npm run build -w @bitlogin/core
+      - run: npm run build -w @bitlogin/widget
+      - run: npm run build -w @bitlogin/demo
+
+      - uses: actions/configure-pages@v5
+
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: packages/demo/dist
+
+  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