dsh-model-slider: Enhanced DeepSeek Harness Model Selector

The `dsh-model-slider` plugin optimizes the model-switching workflow in DSH Web UI through slider and panel integration. It provides low/medium/high quick-switch tiers, advanced selection, multimodal icons, and provider balance monitoring. It takes effect after installation, with its configuration file located at `~/.dsh/.model-slider.json`; it supports membership and prepaid usage queries and securely reads API keys.