feat(desktop): let a pane prefix the zone tab menu

A Browser tab needs one extra verb without replacing Reload / Close. Give
the pane a tabMenuPrefix slot the strip menu renders when it opens.

apps/desktop/src/app/chat/pane-mirror.ts

@@ -10,6 +10,7 @@ import type { ReadableAtom } from 'nanostores'
 import type { ReactElement, ReactNode, PointerEvent as ReactPointerEvent } from 'react'
 
 import { registerPaneCloser, removeTreePane, treePanesWithPrefix } from '@/components/pane-shell/tree/store'
+import type { MenuKit } from '@/components/ui/actions-menu'
 import { registry } from '@/contrib/registry'
 import type { WorkspaceMode } from '@/contrib/types'
 import type { TileDock } from '@/store/session-states'
@@ -52,6 +53,8 @@ export interface PaneMirror<T> {
    *  Per tile so a mirror can offer it for some of its tabs and not others. */
   newTab?: (key: string) => (() => void) | undefined
   render: (key: string) => ReactNode
+  /** Extra rows at the top of the zone tab menu (see PaneChrome.tabMenuPrefix). */
+  tabMenuPrefix?: (key: string) => ((kit: MenuKit) => ReactNode) | undefined
   /** Wrap the tile's TAB (domain context menu — session verbs). */
   tabWrap?: (key: string, tab: ReactElement) => ReactNode
   /** Override the tile's TAB drag (session drop language: stack/split/link).
@@ -113,6 +116,7 @@ export function paneMirror<T>(cfg: PaneMirror<T>): () => void {
           tabDrag: cfg.tabDrag
             ? (event: ReactPointerEvent<HTMLElement>, onTap: () => void) => cfg.tabDrag!(key, event, onTap)
             : undefined, // returns boolean (handled) — see PaneChrome.tabDrag
+          tabMenuPrefix: cfg.tabMenuPrefix?.(key),
           tabWrap: cfg.tabWrap ? (tab: ReactElement) => cfg.tabWrap!(key, tab) : undefined
         },
         render: () => cfg.render(key),

apps/desktop/src/components/pane-shell/tree/renderer/track-model.ts

@@ -9,6 +9,7 @@
 
 import type * as React from 'react'
 
+import type { MenuKit } from '@/components/ui/actions-menu'
 import type { Contribution } from '@/contrib/types'
 
 import type { GroupNode, LayoutNode } from '../model'
@@ -75,6 +76,10 @@ interface PaneChrome extends PaneSizing {
    *  pin/branch/rename/archive/delete). The wrapper must render `tab` as its
    *  interactive child; the zone's own strip menu still owns non-tab space. */
   tabWrap?: (tab: React.ReactElement) => React.ReactNode
+  /** Extra rows at the top of the zone tab menu. Called when the menu opens
+   *  against the right-clicked pane — a Browser tab's Open-in-external, without
+   *  replacing Reload / Close / the strip. */
+  tabMenuPrefix?: (kit: MenuKit) => React.ReactNode
   /** Override this pane's TAB drag (a session tab drags like a sidebar row —
    *  stack / split / composer-link — not the generic pane move). Given the
    *  tab's tap (activate) so that gesture survives. Returns whether it took the

apps/desktop/src/components/pane-shell/tree/renderer/tree-group.tsx

@@ -97,6 +97,7 @@ function ZoneMenu({
   minimized,
   nodeId,
   stripVisible,
+  tabMenuPrefix,
   targetPane
 }: {
   children: ReactNode
@@ -111,6 +112,8 @@ function ZoneMenu({
   /** Whether the strip is on screen — the Hide/Show row toggles against what
    *  the user can see, not against the stored mode (a zone on auto has none). */
   stripVisible?: boolean
+  /** Domain verbs for the right-clicked pane, resolved when the menu opens. */
+  tabMenuPrefix?: (kit: MenuKit) => ReactNode
   /** The right-clicked chip (else the active pane) — what the close-others /
    *  to-the-right / all verbs measure from. Called when the menu RENDERS, not
    *  on every zone re-render: resolving the siblings reads the layout tree,
@@ -131,8 +134,12 @@ function ZoneMenu({
     const paneId = closable?.()
     const targetId = targetPane()
 
+    const prefix = tabMenuPrefix?.(kit)
+
     return (
       <>
+        {prefix}
+        {prefix ? <kit.Separator /> : null}
         {renderActionItem(kit, {
           icon: 'refresh',
           label: t.zones.reload,
@@ -394,6 +401,7 @@ export function TreeGroup({
     minimized: node.minimized,
     nodeId: node.id,
     stripVisible,
+    tabMenuPrefix: (kit: MenuKit) => paneChrome(paneFor(targetPane())).tabMenuPrefix?.(kit),
     targetPane
   }