dsh-later: Adding Scheduled Sending to DSH

`dsh-later` is a plugin for DeepSeek Harness (DSH) designed to enable scheduled message sending and reminders. It allows users to create tasks via the `/later` or `/schedule` commands or interface buttons, supporting delayed execution, scheduled reminders, and multi-segment delay syntax. It also supports server-side triggering after the web page is closed. The plugin provides sidebar status indicators, task list management, and multilingual support. Usage notes include a maximum of 10 tasks per batch, a minimum interval of 60 seconds, and reminders that do not persist across sessions. The plugin offers a convenient capability for background task execution and long-duration reminders.