fix(ui): a disabled menu row says why it is off, under its label

The reason for a disabled menu row lived in a `title` on a natively disabled
button: no pointer events, so no tooltip, and roving focus skips the row —
four translated sentences reached nobody (the Watch later rename/delete
lock, "no other list to move to", the reorder gate). An aria-disabled row was
tried and reverted in E10's review. ActionMenu items now take a `note`,
rendered as a muted line under a disabled row's label: seen, and readable in
a screen reader's browse mode. Only the icon and label dim — row-level
opacity would dim the explanation too. The reorder group states its reason
once, under its first row.

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

@@ -42,6 +42,22 @@ describe("ActionMenu", () => {
 
   // floating-ui never computes a position in jsdom, so the surface stays visibility:hidden; the items
   // are in the DOM but the a11y tree treats them as hidden — hence `{ hidden: true }` on role queries.
+  it("says why a disabled row is off, under its label — and only on a disabled row", () => {
+    // A `title` on a disabled row reached nobody: a disabled button fires no pointer events and
+    // roving focus skips it. The reason is visible text now, undimmed so it can be read.
+    renderMenu([
+      { id: "move", label: "Move to", onSelect: vi.fn(), disabled: true, note: "No other list" },
+      { id: "sync", label: "Sync", onSelect: vi.fn(), note: "never shown" },
+    ]);
+    fireEvent.click(screen.getByRole("button", { name: "Catalog actions" }));
+    const [move, sync] = screen.getAllByRole("menuitem", { hidden: true });
+    expect(move!.textContent).toBe("Move toNo other list");
+    expect(sync!.textContent).toBe("Sync");
+    const note = screen.getByText("No other list");
+    expect(note.closest(".opacity-50")).toBeNull();
+    expect(screen.getByText("Move to").className).toContain("opacity-50");
+  });
+
   it("opens on click and shows one menuitem per item", () => {
     renderMenu([
       { id: "sync", label: "Sync", onSelect: vi.fn() },