Phase 6: Workspaces — Widget CRUD, Manager-Check, Cross-Tenant, Zustand Store, Settings Route

Backend:
- Widget CRUD: get_widgets, create_widget, update_widget, delete_widget
- Manager role check: is_workspace_manager
- Cross-tenant validation: verify_user_same_tenant (UserTenant)
- Default workspace seeding: seed_default_workspace with 12 standard modules
- Set user default workspace: set_user_default_workspace
- Fix create_workspace default uniqueness (unset others before insert)
- Widget CRUD routes: GET/POST/PUT/DELETE /{workspace_id}/widgets
- Set-default route: POST /{workspace_id}/set-default
- Cross-tenant validation in assign_user route

Frontend:
- workspaceStore (Zustand): central state with sessionStorage persistence
- API client interceptor: X-Workspace-ID header on all requests
- useWorkspace hook refactored to use workspaceStore
- Widget API hooks: useWorkspaceWidgets, useCreateWorkspaceWidget, etc.
- useSetDefaultWorkspace hook
- Settings route: /settings/workspaces with WorkspaceManagerPage
- Settings nav item for Workspaces

Tests:
- 25 backend tests (CRUD, modules, widgets, users, manager, seeding, context, isolation)
- 12 frontend tests (workspaceStore state, visibility, persistence, reset)
- 48/48 backend tests passing
- 12/12 frontend tests passing

frontend/src/routes/index.tsx

@@ -64,6 +64,7 @@ const CustomFieldsPage = React.lazy(() => import('@/pages/CustomFields').then(m
 const ActivityTimelinePage = React.lazy(() => import('@/pages/ActivityTimeline').then(m => ({ default: m.ActivityTimelinePage })));
 const SettingsWebhooksPage = React.lazy(() => import('@/pages/SettingsWebhooks').then(m => ({ default: m.SettingsWebhooksPage })));
 const SettingsBackupPage = React.lazy(() => import('@/pages/SettingsBackup').then(m => ({ default: m.SettingsBackupPage })));
+const WorkspaceManagerPage = React.lazy(() => import('@/pages/SettingsWorkspaces').then(m => ({ default: m.WorkspaceManagerPage })));
 const SettingsRechtePage = React.lazy(() => import('@/pages/SettingsRechte').then(m => ({ default: m.SettingsRechtePage })));
 const NoAccessPage = React.lazy(() => import('@/pages/NoAccessPage').then(m => ({ default: m.NoAccessPage })));
 const StartPage = React.lazy(() => import('@/pages/StartPage').then(m => ({ default: m.StartPage })));
@@ -193,6 +194,7 @@ const router = createBrowserRouter([
           { path: 'menu', element: withSuspense(<SettingsMenuOrderPage />) },
           { path: 'custom-fields', element: withSuspense(<CustomFieldsPage />) },
           { path: 'webhooks', element: withSuspense(<SettingsWebhooksPage />) },
+          { path: 'workspaces', element: withSuspense(<WorkspaceManagerPage />) },
           { path: 'backup', element: withSuspense(<SettingsBackupPage />) },
           { path: 'rechte', element: <PermissionRoute permission="settings:read">{withSuspense(<SettingsRechtePage />)}</PermissionRoute> },
           { path: '*', element: <PluginRouteRenderer /> },