Add instance experimental setting for isolated workspaces

Introduce a singleton instance_settings store and experimental settings API, add the Experimental instance settings page, and gate execution workspace behavior behind the new enableIsolatedWorkspaces flag.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/routes/index.ts

@@ -12,3 +12,4 @@ export { dashboardRoutes } from "./dashboard.js";
 export { sidebarBadgeRoutes } from "./sidebar-badges.js";
 export { llmRoutes } from "./llms.js";
 export { accessRoutes } from "./access.js";
+export { instanceSettingsRoutes } from "./instance-settings.js";