feat(design-system): ThemeProvider + useTheme — toggle dark/light (Sprint 0.5 étape 2)

src/shared/lib/theme.ts

@@ -0,0 +1,26 @@
+import { createContext } from 'react'
+
+export type Theme = 'light' | 'dark'
+
+export interface ThemeContextValue {
+  theme: Theme
+  setTheme: (t: Theme) => void
+}
+
+export const ThemeContext = createContext<ThemeContextValue | null>(null)
+
+const STORAGE_KEY = 'expria-theme'
+
+export function getInitialTheme(): Theme {
+  const stored = localStorage.getItem(STORAGE_KEY)
+  if (stored === 'light' || stored === 'dark') return stored
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+}
+
+export function applyTheme(theme: Theme): void {
+  document.documentElement.classList.toggle('dark', theme === 'dark')
+}
+
+export function persistTheme(theme: Theme): void {
+  localStorage.setItem(STORAGE_KEY, theme)
+}