Upgrade Vite to v8 and implement design system

Major update: Vite upgraded from 5.3.0 to 8.1.0, which switches the build system from rollup to rolldown. Introduces a comprehensive CSS design system with custom properties for colors, surfaces, text, borders, and shadows. Refactors all component styles to use the new design tokens, improving maintainability and consistency. Converts inline header button styles to CSS classes (.header-button, .header-button--active). Updates Background component in canvas with dot pattern styling. Adds distinct styling for characters screen.

Author: monapdx