feat(client): browser-based web UI to drive agents from any device (incl. mobile)

## Problem

The only human client today is the Tauri desktop app, so I can only use the workspace at a machine where it's installed. The relay is already headless and self-hostable -- I run it on a Linux server with no GUI -- but there's no way to actually use it from that server, or from my phone.

Agents don't keep office hours. When one posts a review or hits an approval gate at 11pm, I want to glance at my phone, read the thread, and approve -- not be tied to the one desktop where the app lives. Steering agents from anywhere is exactly what an agent workspace should nail, and right now the client is the bottleneck.

## Proposal

A browser-accessible, mobile-friendly web UI that talks to the relay over the same WebSocket + REST surface the desktop app already uses -- enough to read channels, reply, react, and approve/deny workflow gates from any device.

I know the current React/Vite frontend is coupled to Tauri APIs (`invoke`, native binaries like `buzz-acp`), so it can't just be served in a browser as-is. A lighter web client focused on the read + reply + approve core would already cover most of the need; git hosting and local shell tools can stay desktop-only.

## Why it matters

- Self-hosters get a usable client on the same box as the relay.
- Mobile access turns async agent work into "approved it from my phone."
- Share a URL instead of a per-platform desktop install.

Happy to help test on self-hosted Linux + mobile.

Start by inspecting the current React/Vite frontend and its Tauri `invoke` calls, then map the relay's existing WebSocket and REST surface. Define the smallest browser client for reading channels, replying, reacting, and approving or denying workflow gates; done means those flows work from a browser on desktop and mobile without the native `buzz-acp` binaries.

Written by the indexing model from the issue text.