feat: virtual desktop settings toggle backend (#23171)

Adds a new `site_config` entry that controls whether the virtual desktop
feature for Coder Agents is enabled. It can be set via a new
`/api/experimental/chats/config/desktop-enabled` endpoint, which will be
used by the frontend.

coderd/chats.go

@@ -2554,6 +2554,49 @@ func (api *API) putChatSystemPrompt(rw http.ResponseWriter, r *http.Request) {
 	rw.WriteHeader(http.StatusNoContent)
 }
 
+// EXPERIMENTAL: this endpoint is experimental and is subject to change.
+//
+//nolint:revive // get-return: revive assumes get* must be a getter, but this is an HTTP handler.
+func (api *API) getChatDesktopEnabled(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	enabled, err := api.Database.GetChatDesktopEnabled(ctx)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error fetching desktop setting.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+	httpapi.Write(ctx, rw, http.StatusOK, codersdk.ChatDesktopEnabledResponse{
+		EnableDesktop: enabled,
+	})
+}
+
+// EXPERIMENTAL: this endpoint is experimental and is subject to change.
+func (api *API) putChatDesktopEnabled(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	if !api.Authorize(r, policy.ActionUpdate, rbac.ResourceDeploymentConfig) {
+		httpapi.Forbidden(rw)
+		return
+	}
+
+	var req codersdk.UpdateChatDesktopEnabledRequest
+	if !httpapi.Read(ctx, rw, r, &req) {
+		return
+	}
+	if err := api.Database.UpsertChatDesktopEnabled(ctx, req.EnableDesktop); httpapi.Is404Error(err) {
+		httpapi.ResourceNotFound(rw)
+		return
+	} else if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error updating desktop setting.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+	rw.WriteHeader(http.StatusNoContent)
+}
+
 // EXPERIMENTAL: this endpoint is experimental and is subject to change.
 //
 //nolint:revive // get-return: revive assumes get* must be a getter, but this is an HTTP handler.