style(App): update navigation icons for improved clarity

- Replaced navigation icons in App.vue to better represent their respective sections, enhancing user experience and visual consistency.

frontend/src/App.vue

@@ -110,15 +110,15 @@
             <div class="nav-section">
               <h4 class="nav-title">{{ $t('navigation.settings') }}</h4>
               <router-link v-if="isAdmin" to="/club-settings" class="nav-link" title="Vereinseinstellungen">
-                <span class="nav-icon">⚙️</span>
+                <span class="nav-icon">🏛️</span>
                 {{ $t('navigation.clubSettings') }}
               </router-link>
               <router-link v-if="hasPermission('predefined_activities', 'read')" to="/predefined-activities" class="nav-link" title="Vordefinierte Aktivitäten">
-                <span class="nav-icon">⚙️</span>
+                <span class="nav-icon">🎯</span>
                 {{ $t('navigation.predefinedActivities') }}
               </router-link>
               <router-link v-if="hasPermission('teams', 'read')" to="/team-management" class="nav-link" title="Team-Verwaltung">
-                <span class="nav-icon">👥</span>
+                <span class="nav-icon">🧩</span>
                 {{ $t('navigation.teamManagement') }}
               </router-link>
             </div>