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

sis-server.cabal

@@ -39,6 +39,8 @@ library
     , beeline-routing
     , bytestring
     , containers
+    , directory
+    , filepath
     , http-types
     , json-fleece-aeson
     , json-fleece-core
@@ -75,6 +77,8 @@ executable sis-server
     , beeline-routing
     , bytestring
     , containers
+    , directory
+    , filepath
     , http-types
     , json-fleece-aeson
     , json-fleece-core
@@ -114,6 +118,8 @@ test-suite sis-server-test
     , beeline-routing
     , bytestring
     , containers
+    , directory
+    , filepath
     , hspec
     , http-types
     , json-fleece-aeson