build: add Fly.io deployment (nginx static serving)

- Dockerfile: serve the static PWA via nginx:alpine on port 8080
- deploy/nginx.conf: mirror .htaccess (jsx/mjs MIME, no-cache for shell/sw/manifest/meta/dist,
  long-immutable cache for assets, security headers, SPA fallback)
- fly.toml: internal_port 8080, force_https, scale-to-zero
- .dockerignore: exclude .git/node_modules/tests from the image

.dockerignore

@@ -0,0 +1,11 @@
+.git
+.github
+node_modules
+tools
+tests
+Dockerfile
+.dockerignore
+fly.toml
+*.log
+.DS_Store
+**/.DS_Store