feat(frontend): implement basic UI foundation (Issue #6)

frontend/src/context/ThemeContext.tsx

@@ -0,0 +1,42 @@
+import { createContext, useContext, useEffect, useState, ReactNode } from 'react'
+
+type Theme = 'dark' | 'light'
+
+interface ThemeContextType {
+  theme: Theme
+  toggleTheme: () => void
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined)
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setTheme] = useState<Theme>(() => {
+    const saved = localStorage.getItem('theme')
+    return (saved as Theme) || 'dark'
+  })
+
+  useEffect(() => {
+    const root = window.document.documentElement
+    root.classList.remove('light', 'dark')
+    root.classList.add(theme)
+    localStorage.setItem('theme', theme)
+  }, [theme])
+
+  const toggleTheme = () => {
+    setTheme(prev => prev === 'dark' ? 'light' : 'dark')
+  }
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  )
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext)
+  if (context === undefined) {
+    throw new Error('useTheme must be used within a ThemeProvider')
+  }
+  return context
+}