feat(theme): establish design tokens, light/dark mode, and user theme override hooks

[ibourgeois]

Goal

Define the initial visual system for the app with clean spacing, restrained surfaces, and support for theming.

Scope

• Add Tailwind v4 design tokens with CSS variables
• Implement light and dark mode support
• Add the first persistence hook for per-user color overrides
• Reflect the preferred minimal visual language in shared primitives

Out of Scope

• Full profile settings UI
• Detailed brand exploration beyond the initial token system

Acceptance Criteria

• The app has shared design tokens for color, spacing, and surface treatment
• Light and dark themes are supported consistently
• There is a clear place in the data model and UI architecture for user-specific color overrides

Dependencies

• Depends on feat(shell): create application shell and navigation architecture #4

Test Plan

• Add tests where behavior is server-driven
• Manually verify theme switching and token-driven styling on shared screens