dsh-custom-fonts: Customize DeepSeek Harness Web UI Fonts and Font Sizes

`dsh-custom-fonts` is a pure client-side plugin for DeepSeek Harness (DSH), designed to address the inflexibility of font configuration in the Web UI. It allows users to customize UI fonts (separate settings for Chinese and English), code fonts, and global font sizes through a visual "Fonts" settings page without restarting the process. Implemented via CSS style injection, it uses `calc(1em + Npx)` to ensure layout stability. Developers can preview changes in real time and reset them with a single click for the best reading experience.