Replace the Vite template frontend/README.md with an OpenBot frontend guide

[regnull]

Goal

Replace frontend/README.md, which is still the stock "React + TypeScript + Vite" template text, with a short guide to OpenBot's frontend.

Context

Nothing in that file describes this project. A newcomer opening frontend/ should learn where things live: src/pages/, src/components/, the ui.tsx primitives and semantic theme tokens, electron/ (main process, preload, app:// scheme), and how to run it (make dev, pnpm test, pnpm lint, pnpm typecheck). The root README has an Electron section; link to it rather than repeating it.

Acceptance criteria

• The template text is gone
• A one-screen overview of the directory layout and the dev/test commands
• Every command and path is checked against the current tree
• Links to the root README's Desktop app (Electron) section and to CONTRIBUTING.md