[FE-28] Build Dark Mode Theme Toggle with next-themes and hydration guard

[devwums]

Problem

The dark mode toggle does not persist across page navigations or browser sessions. The selected theme resets to light mode on every page load.

Proposed Solution

Create frontend/cntr/ThemeToggle/ThemeToggle.tsx. Uses next-themes (already installed) useTheme() hook. Renders a button toggling between Sun and Moon icons from lucide-react. Guards against hydration mismatch by rendering nothing until mounted is true. All implementation must live inside frontend/cntr/.

Acceptance Criteria

• File at frontend/cntr/ThemeToggle/ThemeToggle.tsx
• Uses next-themes useTheme hook — no manual localStorage reads
• Shows Sun icon in dark mode, Moon icon in light mode
• Renders null (or a placeholder) until mounted is true to prevent hydration mismatch
• Test in frontend/cntr/ThemeToggle/ThemeToggle.test.tsx