dsh-plugin-qr-connect: Add QR code connection entry for DSH Web

`dsh-plugin-qr-connect` is an MIT-licensed plugin for the DSH Web interface, designed to address authentication and connectivity issues when accessing the local DSH Web UI via mobile devices. The plugin adds a QR code button to the sidebar, generating a panel featuring QR codes for both local networks and the public internet. Its core mechanism involves running an authenticated reverse proxy subprocess, responsible for verifying the auth secret, issuing session cookies, and forwarding traffic to the loopback Web UI, while also supporting WebSocket upgrades to display real-time updates. The plugin supports automatic secret rotation (default 30 seconds, configurable to be disabled) and a multilingual UI. Users can install and restart `dsh web` via the `dsh plugin add` command.