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/LanguageSelector.tsx

@@ -0,0 +1,24 @@
+import { Globe } from 'lucide-react';
+import { setLocale, supportedLocales, localeLabels } from '../lib/i18n';
+import { useLocale } from '../hooks/useLocale';
+
+export function LanguageSelector() {
+  const current = useLocale();
+
+  const cycle = () => {
+    const idx = supportedLocales.indexOf(current);
+    const next = supportedLocales[(idx + 1) % supportedLocales.length];
+    setLocale(next);
+  };
+
+  return (
+    <button
+      onClick={cycle}
+      className="flex items-center gap-1.5 rounded-2xl border border-white/8 bg-zinc-800/30 px-2.5 py-1.5 text-xs text-zinc-400 hover:text-zinc-200 hover:bg-white/5 transition-colors"
+      title="Change language"
+    >
+      <Globe size={14} />
+      <span className="font-medium">{localeLabels[current] || current.toUpperCase()}</span>
+    </button>
+  );
+}