feat: consolidate AI settings into one page with tabs

- New SettingsAI page with 3 tabs: KI Assistent, Proaktive KI, MCP
- Remove individual AI/proactive/mcp from settings nav
- Settings nav now 6 items: Stammdaten, Nutzerverwaltung, System, Mail, KI Einstellungen, Benachrichtigungen

frontend/src/pages/Settings.tsx

@@ -19,10 +19,8 @@ export function SettingsPage() {
     { to: '/settings/user-management', label: 'Nutzerverwaltung', icon: '\ud83d\udc65' },
     { to: '/settings/system', label: 'System', icon: '\u2699\ufe0f' },
     { to: '/settings/mail', label: t('mail.settings'), icon: '\ud83d\udce7' },
-    { to: '/settings/ai', label: t('nav.aiAssistant'), icon: '\ud83e\udde0' },
-    { to: '/settings/ai-proactive', label: 'Proaktive KI', icon: '\ud83e\udd16' },
+    { to: '/settings/ai-settings', label: 'KI Einstellungen', icon: '\ud83e\udde0' },
     { to: '/settings/notifications', label: t('settings.notifications'), icon: '\ud83d\udd14' },
-    { to: '/settings/mcp', label: t('settings.mcp', 'MCP'), icon: '\ud83d\udd27' },
   ];
 
   const existingPaths = new Set(hardcodedNavItems.map(item => item.to));