dsh-sleep-send: Adding Scheduled Sending to DSH Web Input Fields

`dsh-sleep-send` is a Cordis client plugin for DSH Web, designed to address the need for "compose messages in advance and send them later." It adds a scheduled-send button and configuration panel on the right side of the input box, allowing messages to be queued for intelligent time slots (e.g., 12:00-14:00, 18:00-08:00 the next day) or custom-specified dates and times, with automatic sending at the scheduled moment. The plugin is a pure ESM zero-dependency package that runs in browser environments, prioritizing the cordis timer service and falling back to browser timers when unavailable. Core features include multi-task queue management, real-time countdown display, and localStorage-based persistent storage, which automatically restores unexpired tasks after page refresh, resends tasks expiring within 15 minutes, and discards those expiring earlier. The plugin also incorporates an anti-accidental-overwrite mechanism.