dsh-stt-input: Add speech input for DSH Web GUI

`dsh-stt-input` is a voice input plugin for the DeepSeek Harness (DSH) Web GUI, designed to address the need for quickly inputting long prompts via voice within DSH. The plugin allows users to record by clicking the microphone button and fills the recognized text directly into the input box upon stopping. It provides two recognition engines: one is browser-based local recognition using the Web Speech API, supporting zero-configuration usage in Chrome/Edge with real-time intermediate results; the other is API recognition based on MediaRecorder and OpenAI-compatible interfaces, supporting OpenAI, Groq, or custom services and multiple Whisper models. The plugin supports configuring service presets, API keys, language, and write mode (append or replace). API