test(channels): cover ActionMenu keyboard-skip + stats ETA value; drop redundant disabledIndices

Deep-review follow-up on R13 S3.

- Add a roving-keyboard-nav test proving a disabled ActionMenu item is
  skipped by arrow keys, and assert the stats "left" row renders the
  formatEta value (formatEta had no coverage anywhere).
- Remove the disabledIndices plumbing from AnchoredPopover + ActionMenu:
  it was redundant. useListNavigation already skips items via their DOM
  disabled/aria-disabled attribute, and PopoverItem renders a native
  disabled button — verified against @floating-ui/react 0.27
  isListIndexDisabled and by a mutation test. Behaviour is unchanged and
  now guarded by the new test.

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

@@ -73,6 +73,43 @@ describe("ActionMenu", () => {
     expect(onSelect).not.toHaveBeenCalled();
   });
 
+  it("skips a disabled item in roving keyboard nav", () => {
+    // items[0] disabled, items[1] + items[2] enabled. Roving keyboard focus (the `tabindex=0` item,
+    // which tracks useListNavigation's activeIndex) must NEVER land on the disabled index-0 — arrowing
+    // skips straight over it. The skip comes for free: PopoverItem renders a native `disabled` button
+    // and useListNavigation reads that DOM state (no explicit disabledIndices needed). Without it,
+    // ArrowUp/Down would strand roving focus on the inert index-0 button. (floating-ui wires this on
+    // real key nav, not on open, in jsdom — so we drive it with arrow keys.)
+    renderMenu([
+      { id: "sync", label: "Sync", onSelect: vi.fn(), disabled: true },
+      { id: "backfill", label: "Backfill", onSelect: vi.fn() },
+      { id: "reset", label: "Reset", onSelect: vi.fn() },
+    ]);
+    fireEvent.click(screen.getByRole("button", { name: "Catalog actions" }));
+    const [disabled, backfill, reset] = screen.getAllByRole("menuitem", { hidden: true }) as [
+      HTMLElement,
+      HTMLElement,
+      HTMLElement,
+    ];
+    const menu = screen.getByRole("menu", { hidden: true });
+    const rover = () =>
+      [disabled, backfill, reset].find((el) => el.getAttribute("tabindex") === "0")?.textContent;
+
+    // The disabled item never becomes the roving target, whichever way we arrow.
+    fireEvent.keyDown(menu, { key: "ArrowDown" }); // skips disabled[0] -> first enabled
+    expect(rover()).toBe("Backfill");
+    expect(disabled.getAttribute("tabindex")).toBe("-1");
+
+    fireEvent.keyDown(menu, { key: "ArrowDown" });
+    expect(rover()).toBe("Reset");
+
+    fireEvent.keyDown(menu, { key: "ArrowUp" }); // back to Backfill
+    expect(rover()).toBe("Backfill");
+    fireEvent.keyDown(menu, { key: "ArrowUp" }); // wraps over disabled[0] to the last item
+    expect(rover()).toBe("Reset");
+    expect(disabled.getAttribute("tabindex")).toBe("-1"); // still never the disabled item
+  });
+
   it("honours a custom trigger node", () => {
     renderMenu([{ id: "sync", label: "Sync", onSelect: vi.fn() }]);
     cleanup();