Adopt vanilla-extract for styling

Summary

Adopt vanilla-extract (VE) as the styling approach for policyengine-app-v2, replacing inline styles and style props with type-safe, zero-runtime CSS-in-JS.

Motivation

• Type safety: Styles are TypeScript — typos and invalid values caught at build time
• Zero runtime: Styles compile to static CSS, no runtime overhead from inline style objects
• Clean JSX: Component files contain logic and markup only; styles live in co-located .css.ts files
• Scalability: As the app grows, .css.ts files are easier to maintain than scattered style props
• Theme contracts: createThemeContract provides type-safe access to design tokens via CSS variables

Approach

1. Install @vanilla-extract/css and @vanilla-extract/vite-plugin
2. Optionally use @mantine/vanilla-extract for Mantine component integration
3. Migrate incrementally — new components use VE, existing components migrated over time
4. Design tokens from @policyengine/design-system become a VE theme contract

What stays the same

• Mantine components (Card, Table, Grid, etc.) — still used for layout/UI
• @policyengine/design-system — still the source of truth for tokens
• Plotly charts — still configured via JS layout objects (VE doesn't apply here)

References

• vanilla-extract docs
• Mantine + vanilla-extract

The issue names no files, tests, or entry points. Start by reviewing the existing styling setup and the vanilla-extract and Mantine integration references, then define the token theme contract and migration boundary. Done means the dependencies are integrated, new styling uses co-located .css.ts files, and the incremental migration plan is established.