dsh-composer-expand: Adding Composer Expand/Collapse to DeepSeek Harness

dsh-composer-expand is a pure frontend plugin developed specifically for the DeepSeek Harness (DSH) Web version, aimed at addressing the issues of limited textarea height and the need for frequent scrolling when inputting long prompts. This plugin provides an expand/collapse button in the Composer toolbar, allowing users to switch between the default height and a 70vh writing view, with a minimum height of 300px in the expanded state. Its core advantages include: pure frontend operation with no new Host commands, no LLM calls, and no session log recording; stable attribute hooks that are not dependent on build hashes; support for Chinese and English localization; and the use of localStorage to persist the expanded state. In terms of interaction, when expanded, a regular Enter key only inserts a line break, and sending requires pressing Ctrl/Cmd+Enter.