feat: product website for GitHub Pages + author CodingPlan.Site

- website/: zero-dependency static landing page (HTML+CSS), corporate
  light theme, real app screenshots, download links pointing to the
  git.codingplan.site release channel (exe/msi) with GitHub mirror
- .github/workflows/deploy-website.yml: deploy website/ to GitHub Pages
  on push to main (paths: website/**)
- package.json: author -> CodingPlan.Site
- README: link to the site and its source folder

.github/workflows/deploy-website.yml

@@ -0,0 +1,31 @@
+name: Deploy website to GitHub Pages
+
+on:
+  push:
+    branches: [main]
+    paths: ['website/**']
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/configure-pages@v5
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: website
+      - id: deployment
+        uses: actions/deploy-pages@v4