fix(ui): a closing dialog leaves the focus to the dialog standing above it

Round 6 widened dialog focus ownership to count focus inside ANY `aria-modal` subtree as ours, so a
dialog and the dialog stacked on it closing together would still hand the focus back. That clause has
no way to tell a layer closing WITH us from one that stays open, so a dialog closing on its own — an
in-flight mutation's `onSuccess`, not a click, since a scrim covers it — under a modal raised above it
pulled the focus onto its own pre-open trigger, behind that scrim, where Enter still activated it.
Three verifiers confirmed the mechanism and two corrected the severity down: it self-heals on the
next Tab, which is why this is a containment violation rather than a broken end state.

`lib/layerStack` cannot refine it. Measured against the real hook: at the instant the lower dialog's
focus cleanup runs, the both-closing case and the sibling-stays-open case give identical readings —
`anyLayerAbove` true, the upper layer topmost — because passive cleanups run parent-first and
`useDialogChrome` is declared before the effect that calls `removeLayer`. Gating on the stack turns
round 6's own test red. What separates them is the DOM: React detaches a deleted subtree before
passive cleanups, so a layer closing with us is already disconnected while one that stays open is
not. Both tests are green, and each was mutation-checked against the other.

Verified in Chrome on localdev: opening the cheat sheet from a sidebar button and closing it restores
that button; with the command palette stacked over the cheat sheet, closing the top one leaves the
focus neither on the background trigger nor on <body>, and closing both returns it to the trigger.

Welcome's Lightbox got the trap and the restore in round 6 but not the layer membership the other
shells have, so its Escape and Tab fired from a raw window listener whatever was on top — the shape
the wizard's comment records being bitten by, unreachable today only by a mount ordering nothing
enforces. It joins the stack. Its comment also read as though the four-shell sweep were finished:
`PlayerModal` is still an `aria-modal` surface with no name, no trap and no restore, which is on
ROADMAP.md now rather than implied to be done.

frontend/src/components/ui/Modal.dom.test.tsx

@@ -254,6 +254,43 @@ describe("Modal focus trap (offsetParent stubbed 'visible')", () => {
     document.body.removeChild(trigger);
   });
 
+  it("leaves the focus alone when a dialog above it is still standing", () => {
+    // The other side of the clause above. Counting focus in ANY `aria-modal` subtree as ours cannot
+    // tell a layer closing WITH us from one that stays open, and the layer stack cannot either: at
+    // the moment this cleanup runs the child's `removeLayer` has not fired, so both cases read "a
+    // layer is above me" (measured). So a dialog that closes on its own — an in-flight mutation's
+    // `onSuccess`, not a click, since a scrim covers it — under a modal raised above it yanked the
+    // focus out of that standing dialog onto its own trigger, behind the scrim, where Enter still
+    // activated it (deep-review round 7). Sibling dialogs, not nested: that is the arrangement
+    // `ConfirmProvider` and the app-wide `ErrorDialog` actually use.
+    const trigger = document.createElement("button");
+    document.body.appendChild(trigger);
+    trigger.focus();
+    function Siblings() {
+      const [lower, setLower] = useState(true);
+      return (
+        <>
+          {lower && (
+            <Modal title="lower" onClose={() => {}}>
+              <button onClick={() => setLower(false)}>close lower</button>
+            </Modal>
+          )}
+          <Modal title="upper" onClose={() => {}}>
+            <button>upper-body</button>
+          </Modal>
+        </>
+      );
+    }
+    render(<Siblings />);
+    const upper = screen.getByText("upper-body");
+    upper.focus();
+    // The lower dialog closes by itself while the upper one holds the focus.
+    act(() => void screen.getByText("close lower").click());
+    expect(screen.queryByText("close lower")).toBeNull(); // it really did unmount
+    expect(document.activeElement).toBe(upper);
+    document.body.removeChild(trigger);
+  });
+
   it("leaves the focus alone when the user moved it out of the dialog themselves", () => {
     // The other half of the same rule, and the branch nothing drove: taking focus back from where
     // the user deliberately put it is the rudest thing a closing dialog can do.