Add selfupdate: replace the running program with the newest Gitea release

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

selfupdate/restart_windows.go

@@ -0,0 +1,7 @@
+package selfupdate
+
+import "errors"
+
+func restart(string) error {
+	return errors.New("restarting in place is not supported on Windows")
+}