[M7-T01] Implement theme system

[RichardHightower]

Task: M7-T01

Milestone: M7 - Theming (0.5 day)
Feature: RuleZ UI

Description

Create a theme system with system preference detection and persistence.

Acceptance Criteria

• Create src/stores/uiStore.ts
• Detect system preference on load
• Implement manual toggle
• Persist preference to localStorage

Store Interface

interface UIStore {
  theme: 'light' | 'dark' | 'system';
  setTheme: (theme: 'light' | 'dark' | 'system') => void;
}

Dependencies

• Depends on: M1-T01