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/hooks/useLocale.ts

@@ -0,0 +1,16 @@
+import { useSyncExternalStore } from 'react';
+import { getLocale, onLocaleChange, t as rawT, type TranslationKey } from '../lib/i18n';
+
+/** Re-renders component when locale changes. Returns the current locale string. */
+export function useLocale(): string {
+  return useSyncExternalStore(onLocaleChange, getLocale, getLocale);
+}
+
+/**
+ * Reactive translation hook.
+ * Components using this will automatically re-render when locale changes.
+ */
+export function useT(): (key: TranslationKey) => string {
+  useLocale(); // subscribe to changes
+  return rawT;
+}