docs(architecture): overhaul ASC Web Apps architecture guide

- Replace legacy Node.js server and vanilla JS app details with new React + Vite stack
- Document cake pattern vertical slice structure and domain models
- Describe REST API integration with Hummingbird backend including `_links` normalization
- Introduce plugin system design and usage patterns
- Provide updated development, testing, build, and deployment workflows
- Detail command-center and console app distinctions and add feature development checklist
- Update CSS architecture and tech stack overview

Author: hanrw