@edge-sky/dsh-oauth-adapter: Add OAuth account page for DSH Web profile

`@edge-sky/dsh-oauth-adapter` is an MIT-licensed plugin providing OAuth account pages for the DSH Web profile. It exposes the existing provider login flow of pi-ai by manually mounting the DSH Authorization Service, supporting providers such as OpenAI Codex, GitHub Copilot, Anthropic, Kimi, OpenRouter, and xAI, and covering various login paths like browser callbacks and Device Code. This plugin is only responsible for providing login entry points and does not manage credentials or maintain login state. Users can install it to the web profile and start it via the `dsh plugin` command.