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/skins/shadlike/context-menu.ts

@@ -10,6 +10,7 @@ import {
 import { NgIcon } from '@ng-icons/core';
 import { ContextMenuService, type MenuItem } from '@core/context-menu.service';
 import { FocusService } from '@core/focus.service';
+import { stepIndex } from '@core/menu-nav';
 import { remPx } from '@core/metrics';
 
 /** Rough per-item metrics in REM, used only to decide whether the menu must
@@ -38,6 +39,7 @@ const EDGE_GAP = 6; // px on purpose: a viewport-edge margin, not a UI size
         #menuEl
         role="menu"
         tabindex="-1"
+        [attr.aria-activedescendant]="activeIndex() >= 0 ? 'ctx-item-' + activeIndex() : null"
         (keydown)="onKeydown($event)"
         [style.left.px]="position().x"
         [style.top.px]="position().y"
@@ -50,6 +52,7 @@ const EDGE_GAP = 6; // px on purpose: a viewport-edge margin, not a UI size
             <button
               type="button"
               role="menuitem"
+              [attr.id]="'ctx-item-' + $index"
               [title]="item.tooltip ?? ''"
               [disabled]="!!item.disabled"
               [class.bg-accent]="$index === activeIndex() && !item.disabled"
@@ -163,18 +166,9 @@ export class ContextMenu {
     }
   }
 
-  /** Move the highlight by one selectable item, skipping separators and
-   *  disabled entries, wrapping. */
-  private step(items: MenuItem[], delta: number): void {
-    const count = items.length;
-    if (!count) return;
-    let index = this.activeIndex();
-    for (let hops = 0; hops < count; hops++) {
-      index = (index + delta + count) % count;
-      if (items[index].type !== 'separator' && !items[index].disabled) {
-        this.activeIndex.set(index);
-        return;
-      }
-    }
+  /** Move the highlight by one selectable item (see stepIndex). */
+  private step(items: MenuItem[], delta: 1 | -1): void {
+    const selectable = items.map((i) => i.type !== 'separator' && !i.disabled);
+    this.activeIndex.set(stepIndex(selectable, this.activeIndex(), delta));
   }
 }