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>

ui/src/lib/queryKeys.ts

@@ -69,6 +69,7 @@ export const queryKeys = {
   },
   instance: {
     schedulerHeartbeats: ["instance", "scheduler-heartbeats"] as const,
+    experimentalSettings: ["instance", "experimental-settings"] as const,
   },
   health: ["health"] as const,
   secrets: {