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

Dockerfile

@@ -0,0 +1,15 @@
+# SecureBit.chat is a static PWA (no backend). The committed build artifacts in
+# dist/ are served as-is by nginx — matching the project's release workflow,
+# where dist/ is rebuilt and committed for every release.
+FROM nginx:1.27-alpine
+
+# Replace the default nginx config with our static-serving config.
+COPY deploy/nginx.conf /etc/nginx/nginx.conf
+
+# Serve the repository (src/, assets/, libs/, dist/, config/, logo/, sw.js, ...).
+COPY . /usr/share/nginx/html
+
+# Fly.io health checks and routing target this port.
+EXPOSE 8080
+
+CMD ["nginx", "-g", "daemon off;"]