docs: add Panda to clients list (Desktop and Web)

[lukaisluka]

Adding Panda to the Desktop and Web section (alphabetical, between Ngent and Poolside Desktop Assistant).

• Repo: https://github.com/lukaisluka/Panda (Apache-2.0)
• Live web app: https://lukaisluka.github.io/Panda/ — pure protocol client in the browser (connects to any ACP-over-WebSocket endpoint)
• Scripted in-browser demo, no agent needed: https://lukaisluka.github.io/Panda/#/demo
• Desktop build (macOS/Windows) spawns stdio agents directly

Panda is built for ACP agent developers: you implement the agent side of the protocol, and Panda is the client UI you'd otherwise have to build yourself — streaming messages, tool-call cards, inline permission prompts, polished diffs, and session history, speaking ACP natively. Off-the-shelf agents (Claude Code, Codex, …) work too through their ACP adapters.

The client-side contract Panda holds agents to (required methods, handler semantics, timeouts) is documented for implementers: https://github.com/lukaisluka/Panda/blob/main/docs/acp-agent-requirements.en.md

Happy to adjust the wording or category if it fits elsewhere better.