Mirror of the MCP-clients panel: Add / Remove / Refresh
with last-discovery health (workflow count or error_kind),
sister Add-dialog with name + base_url + optional
api_key_env + notes. HubService gains the four passthroughs
(`listN8nEndpoints`, `refreshN8nEndpoints`,
`addN8nEndpoint`, `removeN8nEndpoint`) and a
`N8nEndpointInfo` data class.
Settings dialog now hosts: hub endpoint, channels,
system AI, MCP clients, n8n endpoints — all behind the
existing 520×640 SingleChildScrollView so the layout
doesn't crowd.
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>