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>

website/serve-local.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Builds the web app, drops it into website/app/, and serves the whole
+# website locally so the /app/ links behave like they do in production.
+set -euo pipefail
+
+PORT="${1:-8000}"
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(dirname "$SCRIPT_DIR")"
+
+echo "Building web app (wasmJs)..."
+"$REPO_ROOT/gradlew" -p "$REPO_ROOT" :webApp:wasmJsBrowserDistribution
+
+echo "Copying web app into website/app/..."
+mkdir -p "$SCRIPT_DIR/app"
+cp -r "$REPO_ROOT/webApp/build/dist/wasmJs/productionExecutable/." "$SCRIPT_DIR/app/"
+
+echo "Serving website at http://localhost:$PORT/ (web app at http://localhost:$PORT/app/)"
+cd "$SCRIPT_DIR" && python3 -m http.server "$PORT"