fix(ai-chat): restore workspace AI chat enable toggle for self-hosted

The EE-frontend removal (a88b3f77) deleted enable-ai-chat.tsx, the only UI
that set the workspace flag settings.ai.chat. The backend gates remained:
the header assistant icon (page-header-menu) and the /ai-chat/stream endpoint
both require settings.ai.chat === true, and the flag is auto-enabled only in
cloud mode. As a result, self-hosted community builds could configure an AI
provider but had no way to turn the chat on, so the assistant icon never
appeared.

Restore a single admin toggle wired to the already-existing backend `aiChat`
field, modeled on the adjacent MCP settings switch:
- add aiChat?: boolean to the client IWorkspace type
- add AiChatSettings component (optimistic update, revert-on-error,
  workspaceAtom sync so the icon updates without reload)
- render it (admin-only) in workspace settings between AI / Models and
  AI / External tools (MCP)

Generative AI and AI search toggles, removed in the same commit, are left
out of scope.

apps/client/src/features/workspace/components/settings/components/ai-chat-settings.tsx

@@ -0,0 +1,61 @@
+import { workspaceAtom } from "@/features/user/atoms/current-user-atom.ts";
+import { useAtom } from "jotai";
+import { useState } from "react";
+import { updateWorkspace } from "@/features/workspace/services/workspace-service.ts";
+import { Switch, Stack } from "@mantine/core";
+import { notifications } from "@mantine/notifications";
+import useUserRole from "@/hooks/use-user-role.tsx";
+import { useTranslation } from "react-i18next";
+
+export default function AiChatSettings() {
+  const { t } = useTranslation();
+  const [workspace, setWorkspace] = useAtom(workspaceAtom);
+  const { isAdmin } = useUserRole();
+
+  const [checked, setChecked] = useState<boolean>(
+    workspace?.settings?.ai?.chat ?? false,
+  );
+  const [isLoading, setIsLoading] = useState(false);
+
+  async function handleToggle(value: boolean) {
+    setIsLoading(true);
+    const previous = checked;
+    setChecked(value); // optimistic update
+    try {
+      const updated = await updateWorkspace({ aiChat: value });
+      // Force settings.ai.chat to the new value so the atom is consistent
+      // even if the response shape omits it.
+      setWorkspace({
+        ...updated,
+        settings: {
+          ...updated.settings,
+          ai: { ...updated.settings?.ai, chat: value },
+        },
+      });
+      notifications.show({ message: t("Updated successfully") });
+    } catch (err) {
+      console.log(err);
+      setChecked(previous); // revert on failure
+      notifications.show({
+        message: t("Failed to update data"),
+        color: "red",
+      });
+    } finally {
+      setIsLoading(false);
+    }
+  }
+
+  return (
+    <Stack mt="sm">
+      <Switch
+        label={t("AI chat")}
+        description={t(
+          "Enable the AI chat assistant so users can have multi-turn conversations with AI about your workspace content.",
+        )}
+        checked={checked}
+        disabled={!isAdmin || isLoading}
+        onChange={(event) => handleToggle(event.currentTarget.checked)}
+      />
+    </Stack>
+  );
+}

apps/client/src/features/workspace/types/workspace.types.ts

@@ -23,6 +23,7 @@ export interface IWorkspace {
   generativeAi?: boolean;
   disablePublicSharing?: boolean;
   mcpEnabled?: boolean;
+  aiChat?: boolean;
   trashRetentionDays?: number;
   restrictApiToAdmins?: boolean;
   allowMemberTemplates?: boolean;

apps/client/src/pages/settings/workspace/workspace-settings.tsx

@@ -3,6 +3,7 @@ import WorkspaceNameForm from "@/features/workspace/components/settings/componen
 import WorkspaceIcon from "@/features/workspace/components/settings/components/workspace-icon.tsx";
 import McpSettings from "@/features/workspace/components/settings/components/mcp-settings.tsx";
 import AiProviderSettings from "@/features/workspace/components/settings/components/ai-provider-settings.tsx";
+import AiChatSettings from "@/features/workspace/components/settings/components/ai-chat-settings.tsx";
 import AiMcpServers from "@/features/workspace/components/settings/components/ai-mcp-servers.tsx";
 import { useTranslation } from "react-i18next";
 import { getAppName } from "@/lib/config.ts";
@@ -36,6 +37,11 @@ export default function WorkspaceSettings() {
 
           <Divider my="lg" />
 
+          <SettingsTitle title={t("AI / Chat")} />
+          <AiChatSettings />
+
+          <Divider my="lg" />
+
           <SettingsTitle title={t("AI / External tools (MCP)")} />
           <AiMcpServers />
         </>