docs: prepare v1.6.4 changelog

Author: mgrimace

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+## [v1.6.4] - 2026-06-02
+
 - Fix: live todo updates now recover missed changes after reconnects, keep backgrounded PWAs connected more reliably, and avoid unnecessary refreshes on resume.
 - Fix: creating, editing, deleting, and clearing tasks now feel instant in the UI while rolling back cleanly if a request fails.