feat(web): useSetVisibility hook + adjacentTransitions helper + MSW handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/objects/transitions.test.ts

@@ -0,0 +1,13 @@
+import { expect, test } from "vitest";
+
+import { adjacentTransitions } from "./transitions";
+
+test("draft can only go forward to internal", () => {
+  expect(adjacentTransitions("draft")).toEqual({ forward: "internal" });
+});
+test("internal can go forward to public and back to draft", () => {
+  expect(adjacentTransitions("internal")).toEqual({ forward: "public", back: "draft" });
+});
+test("public can only go back to internal", () => {
+  expect(adjacentTransitions("public")).toEqual({ back: "internal" });
+});

web/src/objects/transitions.ts

@@ -0,0 +1,14 @@
+export type Visibility = "draft" | "internal" | "public";
+
+/** The legal one-step visibility moves from `v`, per the backend state machine
+ *  (Draft<->Internal, Internal<->Public; no skipping). */
+export function adjacentTransitions(v: Visibility): { forward?: Visibility; back?: Visibility } {
+  switch (v) {
+    case "draft":
+      return { forward: "internal" };
+    case "internal":
+      return { forward: "public", back: "draft" };
+    case "public":
+      return { back: "internal" };
+  }
+}