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.

site/src/api/typesGenerated.ts

@@ -1170,6 +1170,14 @@ export interface ChatCostUsersResponse {
 	readonly users: readonly ChatCostUserRollup[];
 }
 
+// From codersdk/chats.go
+/**
+ * ChatDesktopEnabledResponse is the response for getting the desktop setting.
+ */
+export interface ChatDesktopEnabledResponse {
+	readonly enable_desktop: boolean;
+}
+
 // From codersdk/chats.go
 /**
  * ChatDiffContents represents the resolved diff text for a chat.
@@ -6578,6 +6586,14 @@ export interface UpdateAppearanceConfig {
 	readonly announcement_banners: readonly BannerConfig[];
 }
 
+// From codersdk/chats.go
+/**
+ * UpdateChatDesktopEnabledRequest is the request to update the desktop setting.
+ */
+export interface UpdateChatDesktopEnabledRequest {
+	readonly enable_desktop: boolean;
+}
+
 // From codersdk/chats.go
 /**
  * UpdateChatModelConfigRequest updates a chat model config.