[M7-T02] Create ThemeToggle component

## Task: M7-T02

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

## Description

Create a theme toggle button with sun/moon icons.

## Acceptance Criteria

- [ ] Create `ThemeToggle.tsx`
- [ ] Add sun/moon icons
- [ ] Wire up to uiStore

## Dependencies

- Depends on: M7-T01