Fix static file serving by using fs.Sub on embed FS

The //go:embed static directive nests files under static/, so after
StripPrefix removes /static/ from the URL, the FileServer couldn't find
the files. Use fs.Sub to root the FS at the static/ subdirectory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/httpserver/router.go

@@ -1,6 +1,7 @@
 package httpserver
 
 import (
+	"io/fs"
 	"net/http"
 
 	"provisioning/internal/api"
@@ -25,7 +26,8 @@ func NewRouter(d Deps) http.Handler {
 	r.Use(middleware.Logger)
 
 	// Static files
-	r.Handle("/static/*", http.StripPrefix("/static/", http.FileServer(http.FS(web.Static))))
+	staticFS, _ := fs.Sub(web.Static, "static")
+	r.Handle("/static/*", http.StripPrefix("/static/", http.FileServer(http.FS(staticFS))))
 
 	// SSE
 	r.Get("/events", d.Hub.ServeSSE)