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/ThinkingBlock.tsx

@@ -1,8 +1,9 @@
 import { useState } from 'react';
 import { ChevronRight, ChevronDown, Brain } from 'lucide-react';
-import { t } from '../lib/i18n';
+import { useT } from '../hooks/useLocale';
 
 export function ThinkingBlock({ text }: { text: string }) {
+  const t = useT();
   const [open, setOpen] = useState(false);
 
   return (