Add dark mode toggle via next-themes

Uses next-themes with system default. Toggle button in sidebar
switches between light/dark and persists to localStorage.
CSS variables for dark mode were already defined in globals.css.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/layout/ThemeProvider.tsx

@@ -0,0 +1,11 @@
+'use client'
+
+import { ThemeProvider as NextThemesProvider } from 'next-themes'
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  return (
+    <NextThemesProvider attribute="class" defaultTheme="system" enableSystem disableTransitionOnChange>
+      {children}
+    </NextThemesProvider>
+  )
+}