feat: MCP Server plugin (Task 5.16) - expose LeoCRM tools to external AI clients

- New plugin app/plugins/builtins/mcp_server/ with 9 MCP tools
- Tools: search_contacts, get_contact, create_contact, list_calendar_entries,
  create_calendar_entry, list_emails, send_email, list_files, upload_file
- Routes: GET /api/v1/mcp/tools, POST /api/v1/mcp/tools/{name}/execute, GET /api/v1/mcp/config
- API-token auth via session + RBAC (mcp:read, mcp:write)
- Frontend: mcp.ts API client with React Query hooks
- Frontend: SettingsMcp.tsx settings page with tool listing and execution
- i18n: de.json and en.json updated with MCP entries
- Tests: 7 tests covering tool listing, config, execution, auth, schema validation

frontend/src/pages/Settings.tsx

@@ -22,6 +22,7 @@ export function SettingsPage() {
     { to: '/settings/ai-proactive', label: 'Proaktive KI', icon: '\ud83e\udd16' },
     { to: '/settings/notifications', label: t('settings.notifications'), icon: '\ud83d\udd14' },
     { to: '/settings/theme', label: t('settings.theme', 'Theme'), icon: '\ud83c\udfa8' },
+    { to: '/settings/mcp', label: t('settings.mcp', 'MCP'), icon: '\ud83d\udd27' },
   ];
 
   const existingPaths = new Set(hardcodedNavItems.map(item => item.to));