Documentation: Add frontend startup instructions

[doobidoo]

The current documentation doesn't specify how to start the frontend application properly. This can lead to confusion when users try to run the application.

Suggested additions to the README:

Running the Frontend

After building the backend with cargo build --release, you need to:

1. Start the backend server:

./target/release/deepclaude  # or deepclaude.exe on Windows

2. In a separate terminal, start the frontend development server:

cd frontend
npm install     # Only needed first time
npm run dev     # Preferred for development

Note: While npm start is available, it requires a production build (npm run build). For development purposes, npm run dev is recommended as it provides hot reloading and better error messages.

These instructions will help users get both the backend and frontend running properly.