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/routes/index.tsx

@@ -35,6 +35,7 @@ const AIAssistantPage = React.lazy(() => import('@/pages/AIAssistant').then(m =>
 const AISettingsPage = React.lazy(() => import('@/pages/AISettings').then(m => ({ default: m.AISettingsPage })));
 const ProactiveAISettings = React.lazy(() => import('@/pages/ProactiveAISettings').then(m => ({ default: m.ProactiveAISettings })));
 const SettingsThemePage = React.lazy(() => import('@/pages/SettingsTheme').then(m => ({ default: m.SettingsThemePage })));
+const SettingsMcpPage = React.lazy(() => import('@/pages/SettingsMcp').then(m => ({ default: m.SettingsMcpPage })));
 const AutomationDashboardPage = React.lazy(() => import('@/pages/AutomationDashboard').then(m => ({ default: m.AutomationDashboardPage })));
 const AgentDashboardPage = React.lazy(() => import('@/pages/AgentDashboard').then(m => ({ default: m.AgentDashboardPage })));
 const AutomationSettingsPage = React.lazy(() => import('@/pages/AutomationSettings').then(m => ({ default: m.AutomationSettingsPage })));
@@ -107,6 +108,7 @@ const router = createBrowserRouter([
           { path: 'ai-proactive', element: withSuspense(<ProactiveAISettings />) },
           { path: 'theme', element: withSuspense(<SettingsThemePage />) },
           { path: 'automation', element: withSuspense(<AutomationSettingsPage />) },
+          { path: 'mcp', element: withSuspense(<SettingsMcpPage />) },
           { path: '*', element: <PluginRouteRenderer /> },
         ],
       },