Carry over what only the win7 tree had: middle-click close, ARIA, menu fix

The deleted win7 tree held a few behaviours the modern tree never got:

- Context menu: the first ArrowUp in a fresh menu skipped the last item
  (Delete in the row menu, Disconnect in the tab menu) — stepping from -1
  landed one short. The index math moves to core/menu-nav.ts stepIndex(),
  checked by menu-nav.spec.ts (bun test; verified to fail on the old logic).
  Items get ids and the menu exposes aria-activedescendant, so screen
  readers follow the arrow keys while focus stays on the menu.
- Middle-click closes a tab, as in browsers. auxclick also fires for the
  right button, so only button 1 counts; closeWorkspace already refuses the
  last tab. A middle press is swallowed so WebView2 doesn't start autoscroll
  over the scrollable strip. The hover ✕ stays.
- ARIA: the tab strip is a tablist of role=tab buttons with aria-selected
  (the in-tab ✕ is taken out of the a11y tree and tab order — Ctrl/Cmd+W and
  the tab menu remain the keyboard paths); transfer bars are progressbars
  with value and a name.

Co-Authored-By: Claude <noreply@anthropic.com>

ui/src/app/core/menu-nav.ts

@@ -0,0 +1,15 @@
+/**
+ * Next highlighted menu index: skips unselectable entries and wraps. From -1
+ * (nothing highlighted yet), down lands on the first selectable entry and up
+ * on the LAST — stepping up from -1 directly would land one short of it.
+ * Returns `from` unchanged when nothing is selectable.
+ */
+export function stepIndex(selectable: boolean[], from: number, delta: 1 | -1): number {
+  const count = selectable.length;
+  let index = from < 0 && delta < 0 ? 0 : from;
+  for (let hops = 0; hops < count; hops++) {
+    index = (index + delta + count) % count;
+    if (selectable[index]) return index;
+  }
+  return from;
+}