dsh-aurora-bg: Apply a DeepSeek official website-style dynamic aurora background to DSH Web UI

# Introduction When using DSH for agent development, `dsh web` is one of the most frequently opened interfaces. However, the default background style is quite plain, and staring at it for a long time can become tedious. Adding visual customization to the Web UI usually requires modifying the frontend code yourself, which involves a high barrier to entry and maintenance costs. `dsh-aurora-bg` solves this problem: it turns the dynamic background of the DeepSeek official homepage—featuring a deep blue fluid background, aurora light spots, and a dotted whale—into a DSH plugin. Once installed and the interface is restarted, the background takes effect automatically without the need to modify DSH's core code. # What is this? `dsh-aurora-bg` is a DeepSeek Harness (DSH) Web UI background plugin developed by KUN