Add SPA static file serving from backend

Backend (Server.hs):
- serveStaticOrSpa: serves static files from --static-dir
- SPA fallback: extensionless paths serve index.html
- MIME type mapping for html/css/js/png/svg/ico/woff2

Frontend:
- ES module imports with import map for Mithril CDN
- Build script: tsc + copy index.html + static assets

CLI:
- --static-dir flag (default: frontend/dist)
- scripts/run passes through extra args
- Dockerfile CMD points at /usr/local/share/sis/static

Dockerfile

@@ -10,12 +10,11 @@ RUN mkdir -p /build && date -u '+%Y-%m-%d %H:%M UTC' > /build/build-time
 ADD build/sis-server /usr/local/bin/sis-server
 RUN chmod +x /usr/local/bin/sis-server
 
-# Frontend static files are served separately (e.g. nginx, CDN, or
-# a simple static file server). In development, use `npx serve`.
+# Frontend SPA static files, served by sis-server via --static-dir.
 ADD frontend/dist /usr/local/share/sis/static
 
 ENTRYPOINT ["/usr/bin/tini", "-s", "--"]
 
 EXPOSE 8080
 
-CMD ["/usr/local/bin/sis-server", "--port", "8080"]
+CMD ["/usr/local/bin/sis-server", "--port", "8080", "--static-dir", "/usr/local/share/sis/static"]