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/components/ErrorBoundary.tsx

@@ -40,7 +40,7 @@ export class ErrorBoundary extends Component<Props, State> {
       if (this.props.fallback) return this.props.fallback;
 
       return (
-        <div className="h-dvh flex items-center justify-center bg-[#1e1e24] text-zinc-300 p-6">
+        <div className="h-dvh flex items-center justify-center bg-[var(--pc-bg-base)] text-zinc-300 p-6">
           <div className="max-w-md w-full space-y-4 text-center">
             <div className="text-4xl">💥</div>
             <h1 className="text-xl font-semibold text-zinc-100">