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>

.gitignore

@@ -5,6 +5,7 @@ xcodebuild*.log
 
 .gradle/
 .idea/
+/website/app/
 .*.swp
 .*.swo
 .DS_Store