fix: Fix navigation links and restore LanguageSwitcher

- Add missing nav keys to types.ts, de.ts, en.ts (sessions, buddies, stats, wishlist)
- Add LanguageSwitcher back to authenticated header
- Create /sessions page with SessionList
- Create /buddies page with BuddyList
- Create /stats page with StatsDashboard
- Create /wishlist placeholder page

src/i18n/types.ts

@@ -197,6 +197,10 @@ export type TranslationKeys = {
         activity: string;
         search: string;
         profile: string;
+        sessions: string;
+        buddies: string;
+        stats: string;
+        wishlist: string;
     };
     hub: {
         title: string;