Deploy web app to satsprice.app/app/ and link it from the website

The GitHub Pages workflow now builds the Compose wasmJs web app and
publishes it alongside the marketing site, so it's reachable at
/app/. Add nav/hero/download links to it and list Web among the
supported platforms. Add website/serve-local.sh to reproduce the
production layout locally, and document the setup in README/AGENTS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/pages.yml

@@ -5,6 +5,14 @@ on:
     branches: ["main"]
     paths:
       - "website/**"
+      - "shared/**"
+      - "webApp/**"
+      - "gradle/**"
+      - "gradlew"
+      - "gradlew.bat"
+      - "build.gradle.kts"
+      - "settings.gradle.kts"
+      - "gradle.properties"
       - ".github/workflows/pages.yml"
   workflow_dispatch:
 
@@ -26,12 +34,26 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+      - name: Set up JDK
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "21"
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+      - name: Build web app
+        run: ./gradlew :webApp:wasmJsBrowserDistribution
+      - name: Assemble site
+        run: |
+          mkdir -p _site/app
+          cp -r website/. _site/
+          cp -r webApp/build/dist/wasmJs/productionExecutable/. _site/app/
       - name: Setup Pages
         uses: actions/configure-pages@v5
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3
         with:
-          path: "./website"
+          path: "./_site"
       - name: Deploy to GitHub Pages
         id: deployment
         uses: actions/deploy-pages@v4