feat: theme switcher — dark, light, OLED modes with configurable accent colors

- Add ThemeContext with CSS custom properties for all base colors
- Three theme modes: Dark (default), Light, OLED Black
- Six accent colors: Cyan, Violet, Emerald, Amber, Rose, Blue
- Theme switcher dropdown in header (palette icon)
- Persisted in localStorage
- CSS variables replace hardcoded hex colors in index.css and components
- i18n support (EN/FR) for theme labels

src/main.tsx

@@ -2,12 +2,15 @@ import React from 'react'
 import ReactDOM from 'react-dom/client'
 import App from './App'
 import { ErrorBoundary } from './components/ErrorBoundary'
+import { ThemeProvider } from './contexts/ThemeContext'
 import './index.css'
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <ErrorBoundary>
-      <App />
+      <ThemeProvider>
+        <App />
+      </ThemeProvider>
     </ErrorBoundary>
   </React.StrictMode>,
 )