dsh-sysmon: Add system performance floating window for DSH Web GUI

dsh-sysmon is a Web GUI plugin for DeepSeek Harness (DSH), maintained by 21hbguo. Adhering to the "everything is a plugin" philosophy, it must run within the DSH process. The plugin consists of Host-side metrics collection and Client-side native DOM rendering. It provides a floating window in the bottom-right corner of the interface, displaying real-time information about the host's CPU, memory, disk, network, load, uptime, and GPU status. Core features include an auto-scaling progress bar for network rates, allowing users to view the top 10 processes by network usage on hover; multiple disk mount points display with filtering of virtual file systems; and GPU support for both AMD sysfs and NVIDIA nvidia-smi sampling. The floating window supports dragging, collapsing, and hiding. It also persists its position and automatically pauses updates when the page is invisible.