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/components/QuickActionsGrid.tsx

@@ -0,0 +1,34 @@
+'use client';
+
+import NavButton from './NavButton';
+import { Calendar, Users, BarChart3, Heart } from 'lucide-react';
+import { useI18n } from '@/i18n/I18nContext';
+
+export default function QuickActionsGrid() {
+    const { t } = useI18n();
+
+    return (
+        <div className="grid grid-cols-4 gap-3">
+            <NavButton
+                icon={<Calendar size={22} />}
+                label={t('nav.sessions') || 'Events'}
+                href="/sessions"
+            />
+            <NavButton
+                icon={<Users size={22} />}
+                label={t('nav.buddies') || 'Buddies'}
+                href="/buddies"
+            />
+            <NavButton
+                icon={<BarChart3 size={22} />}
+                label={t('nav.stats') || 'Stats'}
+                href="/stats"
+            />
+            <NavButton
+                icon={<Heart size={22} />}
+                label={t('nav.wishlist') || 'Wishlist'}
+                href="/wishlist"
+            />
+        </div>
+    );
+}