dsh-provider-hub: DSH LLM Provider Hub Plugin

`dsh-provider-hub` is a visual plugin for DSH to manage multiple LLM gateways. It replaces manual editing of YAML files with a card-based console in the left sidebar, supports OpenAI and Anthropic protocols, includes a built-in catalog of major models, and allows custom User-Agent headers to bypass attribution restrictions. The plugin configuration is persisted, and you can use it after restarting DSH following installation.