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>

.docker/selfhost/schema.json

@@ -565,6 +565,11 @@
           "type": "boolean",
           "description": "Only allow users with early access features to access the app\n@default false",
           "default": false
+        },
+        "allowGuestDemoWorkspace": {
+          "type": "boolean",
+          "description": "Whether allow guest users to create demo workspaces.\n@default true",
+          "default": true
         }
       }
     },
@@ -592,6 +597,11 @@
           "type": "string",
           "description": "Allowed version range of the app that allowed to access the server. Requires 'client/versionControl.enabled' to be true to take effect.\n@default \">=0.20.0\"",
           "default": ">=0.20.0"
+        },
+        "allowGuestDemoWorkspace": {
+          "type": "boolean",
+          "description": "Allow guests to access demo workspace.\n@default true",
+          "default": true
         }
       }
     },