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/AnchoredPopover.tsx

@@ -91,9 +91,6 @@ export interface AnchoredPopoverOptions {
    *  lands the roving focus on it instead of the first item — the behaviour a "pick one of these
    *  modes" menu needs. Leave unset for action menus and multi-select toggle lists. */
   selectedIndex?: number | null;
-  /** Indices of items that are disabled (e.g. an action whose mutation is already in flight), so
-   *  roving keyboard nav SKIPS them instead of stranding focus on an unfocusable disabled button. */
-  disabledIndices?: number[];
 }
 
 // Internal: call sites use the inferred hook return, they don't name the type.
@@ -178,7 +175,10 @@ export function useAnchoredPopover(opts: AnchoredPopoverOptions) {
       setHoverIndex(null);
     },
     selectedIndex: opts.selectedIndex ?? null,
-    disabledIndices: opts.disabledIndices,
+    // Disabled items are skipped automatically: with no `disabledIndices`, useListNavigation reads
+    // each item's DOM `disabled`/`aria-disabled` (PopoverItem always renders native `disabled`), so a
+    // pending/inert item is never a roving target. (Verified against @floating-ui/react 0.27
+    // `isListIndexDisabled`.) No explicit index list needed.
     loop: true,
     // Focus the first item on open for BOTH click and keyboard (the default 'auto' skips pointer-opens,
     // which strands focus on the menu container — arrows then can't enter the roving list at all).