Clarify manual installation for Vite and development dependencies

Problem

The README's manual installation guidance does not clearly explain when the npm package must be installed or that bundler-based integrations should install it as a development dependency.

In particular, the Vite section imports react-grab from the application entry point without first telling users to install the package. Following those steps in a Vite project therefore results in an unresolved react-grab import.

React Grab is only loaded when import.meta.env.DEV is true, and the grab init CLI already installs it with -D by default. The Webpack manual installation section currently uses npm install react-grab, which is inconsistent with that development-only usage and the CLI behavior.

Expected documentation

• Add npm install -D react-grab before the Vite import example.
• Use the same development dependency command for Webpack.
• Keep the generated package READMEs and llms.txt content consistent.

No Vite plugin or runtime behavior change is required.