feat: add runtime language selector in header (EN/FR toggle)

- Add LanguageSelector component with globe icon + cycle button
- Refactor i18n to support reactive locale switching via useSyncExternalStore
- Locale priority: localStorage > VITE_LOCALE > navigator.language > 'en'
- All components now use useT() hook for reactive re-rendering on locale change
- Persists choice to localStorage (key: pinchchat-locale)
- No page reload needed — instant switch

src/components/Sidebar.tsx

@@ -1,6 +1,6 @@
 import { MessageSquare, X, Sparkles } from 'lucide-react';
 import type { Session } from '../types';
-import { t } from '../lib/i18n';
+import { useT } from '../hooks/useLocale';
 
 interface Props {
   sessions: Session[];
@@ -11,6 +11,7 @@ interface Props {
 }
 
 export function Sidebar({ sessions, activeSession, onSwitch, open, onClose }: Props) {
+  const t = useT();
   return (
     <>
       {open && <div className="fixed inset-0 bg-black/60 backdrop-blur-sm z-40 lg:hidden" onClick={onClose} />}