swarm-ui: rewire a notification flow by drag or keyboard

The flow editor can now change a flow's shape on the canvas, not only
through the inspector's fields:

- drag a box onto another to move it and its branch there; a drop the
  save would refuse shows the target dashed red and gives the reason;
- a palette of node kinds adds a node below the picked box on click, or
  below any box it is dragged onto; it offers what the controller lists;
- arrow keys walk the tree, M picks a box up and Enter drops it under
  the focused one, Alt+Up/Down reorder a node among its siblings, Delete
  removes a branch; focus follows the edit;
- each box with siblings says when it runs, and the inspector can move
  it earlier or later, since the router runs children in edge order;
- request_reviewer is a second leaf kind beside todo, with a reviewer
  field;
- leaving with unsaved edits asks first, also for in-app links and the
  back button (a router hook runs navigation past a page's guard).

frontend/packages/swarm-ui/test/nav-guard.test.mjs

@@ -0,0 +1,28 @@
+// The guard a page holding unsaved work puts in front of in-app navigation.
+// Run by `npm test`.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { mayNavigate, setNavGuard } from "../src/shell/navGuard.ts";
+
+test("with no guard every move goes ahead", () => {
+  setNavGuard(null);
+  assert.equal(mayNavigate("/alerts"), true);
+});
+
+test("a guard that says no stops the move and is told where it was headed", () => {
+  const asked = [];
+  setNavGuard((to) => {
+    asked.push(to);
+    return false;
+  });
+  assert.equal(mayNavigate("/alerts"), false);
+  assert.deepEqual(asked, ["/alerts"]);
+  setNavGuard(null);
+  assert.equal(mayNavigate("/alerts"), true, "removing it lets moves through");
+});
+
+test("a guard that says yes lets the move through", () => {
+  setNavGuard(() => true);
+  assert.equal(mayNavigate("/jobs"), true);
+  setNavGuard(null);
+});