feat(cloud): add configurable Cloud-only WebUI notice banner

Cloud deployments need a way to tell readers what is happening to the
instance they are using (maintenance windows, service notices) without
shipping a release per notice.

Add a cloud.banner config block (switch, tone, dismissal, link targets and
per-language copy) and expose the resolved banner on the unauthenticated
/api/v1/system/info payload so the WebUI can render it before login. The
banner resolves to null on OSS deployments, so a self-hosted copy of
config.yaml never shows managed-service copy. Copy is plain text; only
absolute http(s) link targets are rendered.

The WebUI renders it once from the root layout, offsets the fixed app shell
and the wizard through --cloud-banner-height, and keeps a dismissal bound to
the notice it dismissed so new copy reappears.

Co-authored-by: multica-agent <[email protected]>

src/langbot/pkg/api/http/controller/groups/system.py

@@ -5,6 +5,7 @@ import sqlalchemy
 
 from .. import group
 from .....utils import constants
+from .....cloud.banner import resolve_cloud_banner
 from .....entity.persistence.metadata import WorkspaceMetadata
 from ...authz import Permission
 from ...context import PrincipalType, RequestContext
@@ -219,6 +220,9 @@ class SystemRouterGroup(group.RouterGroup):
                     'edition': constants.edition,
                     'beta': is_beta,
                     'deployment_mode': 'cloud' if is_cloud else 'oss',
+                    # Cloud-only instance notice. None on OSS deployments so a
+                    # self-hosted copy of config.yaml can never surface it.
+                    'cloud_banner': resolve_cloud_banner(self.ap),
                     'enable_marketplace': self.ap.instance_config.data.get('plugin', {}).get(
                         'enable_marketplace', True
                     ),