dsh-space-ptt: Add a PTT key for DSH voice input

dsh-space-ptt is a client-side plugin maintained by wuyou0423. It adds a push-to-talk (PTT) interaction to DeepSeek Harness's voice input, allowing users to speak by holding the spacebar and automatically sending upon release. This eliminates the multi-step process of clicking the input box, microphone, "insert", and "send" buttons. The plugin globally intercepts the spacebar while preserving standard behavior in scenarios such as drafts, pop-ups, and input method selection. It does not import internal host modules but locates the record, insert text, and send message buttons via aria-labels in the official UI, relying on an external contract during host updates. Configuration is centralized in a few constants at the top of lib/client.js, and changes take effect after refreshing the page. Installation can be done using the `dsh plugin` command to load from GitHub, npm, or a local link directory. After installation, DSH must be restarted, and the official voice input bundle and voice model must be enabled beforehand. The plugin is only effective on the Web/Desktop Web UI and runs with the current DSH process permissions. It is recommended to review the source code and the MIT license before installation.