cmd/hello: split css and js into separate files (#19771)

Move the inline CSS and JS into separate files to be more friendly
to Content Security Policies. ServeHTTP is updated to serve these
assets from the '/static/' path.

Updates tailscale/corp#32398

Signed-off-by: Noel O'Brien <noel@tailscale.com>

cmd/hello/helloserver/helloserver.go

@@ -6,7 +6,7 @@ package helloserver
 
 import (
 	"crypto/tls"
-	_ "embed"
+	"embed"
 	"html/template"
 	"log"
 	"net/http"
@@ -21,6 +21,11 @@ import (
 //go:embed hello.tmpl.html
 var embeddedTemplate string
 
+//go:embed static/*
+var staticFiles embed.FS
+
+var staticHandler = http.FileServerFS(staticFiles)
+
 var tmpl = template.Must(template.New("home").Parse(embeddedTemplate))
 
 // Server is an HTTP server for hello.ts.net.
@@ -116,6 +121,12 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		http.Redirect(w, r, "https://"+host, http.StatusFound)
 		return
 	}
+
+	if strings.HasPrefix(r.RequestURI, "/static/") {
+		staticHandler.ServeHTTP(w, r)
+		return
+	}
+
 	if r.RequestURI != "/" {
 		http.Redirect(w, r, "/", http.StatusFound)
 		return