dsh-qq2007-skin: Provides a revertible QQ 2007 style appearance for DSH Web GUI

`dsh-qq2007-skin` is an independent, non-official appearance plugin maintained by LeemanCheung and licensed under the MIT license. The plugin aims to render the DSH Web GUI in the style of a 2007 blue instant messaging window, while fully preserving native interaction functionalities such as sessions, models, attachments, sending, and settings. Its core features include: accessing 72 native theme tokens via the official API without modifying the installation package or enabling debug ports; offering a switchable and undoable retro appearance, along with optional original Web Audio notification sounds; supporting high contrast, reduced motion modes, and narrow-screen adaptation; and storing user settings solely in browser `localStorage`, with no YAML configuration required. Ensure the environment is DSH `0.1.0-rc.6` before installation.