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/instance-settings.test.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from "vitest";
+import {
+  DEFAULT_INSTANCE_SETTINGS_PATH,
+  normalizeRememberedInstanceSettingsPath,
+} from "./instance-settings";
+
+describe("normalizeRememberedInstanceSettingsPath", () => {
+  it("keeps known instance settings pages", () => {
+    expect(normalizeRememberedInstanceSettingsPath("/instance/settings/experimental")).toBe(
+      "/instance/settings/experimental",
+    );
+    expect(normalizeRememberedInstanceSettingsPath("/instance/settings/plugins/example?tab=config#logs")).toBe(
+      "/instance/settings/plugins/example?tab=config#logs",
+    );
+  });
+
+  it("falls back to the default page for unknown paths", () => {
+    expect(normalizeRememberedInstanceSettingsPath("/instance/settings/nope")).toBe(
+      DEFAULT_INSTANCE_SETTINGS_PATH,
+    );
+    expect(normalizeRememberedInstanceSettingsPath(null)).toBe(DEFAULT_INSTANCE_SETTINGS_PATH);
+  });
+});