UI Foundations: Color System, States, and Accessibility

This issue tracks the foundational UI effort to refine the existing visual language across the app.
The goal is to take the current base colors (light and dark) and expand them into a more cohesive, consistent, and contrast-safe system that components can build on going forward.

**Scope**
- Light and dark color palettes derived from existing base colors
- Primary, secondary, and tertiary action styles with clear states (hover, active, focus, disabled)
- Text colors and hierarchy (headings, body, muted text, links)
- Backgrounds and surface levels (cards, panels, modals, tables)
- Form element states and focus visibility
- Status colors (success, warning, error, info)
- WCAG-aware contrast and accessibility considerations

**Guiding principles**
- Minimalistic but engaging layouts
- Intentional, restrained use of color and iconography
- Avoiding visual noise or competing colors within a component

**Output**
- A shared Figma file documenting UI foundations and usage rules