Add Homebridge UI for Script2: split devices into on_off/stateless sections and include UI assets

[pponce]

Motivation

• Provide a friendly Homebridge Config UI X experience for Script2 by introducing a custom UI and a richer platform schema.
• Separate configuration into explicit on_off_switches and stateless_switches to improve clarity while preserving legacy devices compatibility.
• Ship an embedded UI and server shim so users can edit plugin config from Homebridge UI instead of raw JSON.

Description

• Add a custom UI by including homebridge-ui/public/index.html, homebridge-ui/public/index.js, and a server entry homebridge-ui/server.js that uses HomebridgePluginUiServer.
• Update config.schema.json to enable customUi, add on_off_switches and stateless_switches arrays with full property definitions and validation, and keep a conditional legacy devices array for backward compatibility.
• Revise README.md to document the new UI sections, updated configuration examples, migration notes, installation instructions, and recommended child-bridge guidance.
• Bump package version and add @homebridge/plugin-ui-utils dependency in package.json.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task