feat(core): add allowGuestDemoWorkspace flag to force login (#12779)

https://github.com/user-attachments/assets/41a659c9-6def-4492-be8e-5910eb148d6f

This PR enforces login‑first access (#8716) by disabling or enabling the
guest demo workspace via Admin Server Client Page and redirecting
unauthenticated users straight to `/sign‑in`.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added a configuration option to control whether guest users can create
demo workspaces.
* Updated server and client interfaces, GraphQL schema, and queries to
support the new guest demo workspace flag.

* **Bug Fixes**
* Improved sign-out behavior to redirect users appropriately based on
guest demo workspace permissions.
* Enhanced navigation flow to handle guest demo workspace access and
user authentication state.

* **Tests**
* Added tests to verify sign-out logic when guest demo workspaces are
enabled or disabled.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: liuyi <forehalo@gmail.com>
Co-authored-by: fengmk2 <fengmk2@gmail.com>

packages/backend/server/src/schema.gql

@@ -1585,6 +1585,9 @@ enum SearchTable {
 }
 
 type ServerConfigType {
+  """Whether allow guest users to create demo workspaces."""
+  allowGuestDemoWorkspace: Boolean!
+
   """fetch latest available upgradable release of server"""
   availableUpgrade: ReleaseVersionType