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

test/Spec.hs

@@ -1,5 +1,4 @@
-{- | Test entry point for sis-server.
--}
+-- | Test entry point for sis-server.
 module Main (main) where
 
 import Test.Hspec (Spec, describe, hspec, it, shouldBe)
@@ -8,6 +7,7 @@ main :: IO ()
 main = hspec spec
 
 spec :: Spec
-spec = describe "Sis.Server" $
-  it "health check returns ok" $
-    True `shouldBe` True
+spec =
+    describe "Sis.Server" $
+        it "health check returns ok" $
+            True `shouldBe` True