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/database/queries/siteconfig.sql

@@ -140,3 +140,23 @@ SELECT
 -- name: UpsertChatSystemPrompt :exec
 INSERT INTO site_configs (key, value) VALUES ('agents_chat_system_prompt', $1)
 ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_system_prompt';
+
+-- name: GetChatDesktopEnabled :one
+SELECT
+	COALESCE((SELECT value = 'true' FROM site_configs WHERE key = 'agents_desktop_enabled'), false) :: boolean AS enable_desktop;
+
+-- name: UpsertChatDesktopEnabled :exec
+INSERT INTO site_configs (key, value)
+VALUES (
+    'agents_desktop_enabled',
+    CASE
+        WHEN sqlc.arg(enable_desktop)::bool THEN 'true'
+        ELSE 'false'
+    END
+)
+ON CONFLICT (key) DO UPDATE
+SET value = CASE
+    WHEN sqlc.arg(enable_desktop)::bool THEN 'true'
+    ELSE 'false'
+END
+WHERE site_configs.key = 'agents_desktop_enabled';