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

package.yaml

@@ -35,6 +35,8 @@ dependencies:
   - beeline-routing
   - bytestring
   - containers
+  - directory
+  - filepath
   - http-types
   - json-fleece-aeson
   - json-fleece-core