feat: harden support, localize knowledge and add operator availability [skip ci]

Source-only publication of reviewed commit 64e96a7f423673dc29ad56485be9fc79bce8af1c.

Passed in isolation: focused frontend/backend tests, race checks, TypeScript typecheck, lint, Go compilation, Flowgram build, and deterministic SDK build/tests.

Not accepted as a production release: Next.js export/native assembly remains blocked by environment memory limits; browser interaction and MySQL staging acceptance remain unverified. No production server, database/schema, runtime configuration, deployment, release tag, or workflow execution was performed.

Expected source tree: 0a4a7528e9.

internal/builders/conversation_builder.go

@@ -1,3 +1,5 @@
+// Modified in the independent hardening fork; see docs/hardening.
+
 package builders
 
 import (
@@ -45,9 +47,7 @@ func BuildConversationWithLocale(item *models.Conversation, locale string) respo
 		ClosedBy:                  item.ClosedBy,
 		CloseReason:               item.CloseReason,
 	}
-	if identity := services.ConversationService.GetConversationExternalIdentity(item); identity != nil {
-		ret.CustomerOnline = services.WsService.IsGuestOnline(identity.ExternalID)
-	}
+	ret.CustomerOnline = services.WsService.IsCustomerOnline(item.CustomerID, item.ChannelID)
 	if item.CurrentAssigneeID > 0 {
 		if user := services.UserService.Get(item.CurrentAssigneeID); user != nil {
 			ret.CurrentAssigneeName = user.Nickname