feat(header): Alt+W closes the active session tab, customizable in Settings

The close-with-neighbour-activation logic moved into a shared helper
(the strip and the shortcut use the same path). Alt+W is the default
because the browser owns Cmd/Ctrl+W on web; desktop users can rebind it
— the action is registered as customizable, so it appears in the
Settings shortcuts section and the shortcuts help automatically. VS
Code is excluded (it has no session tabs).

packages/ui/src/hooks/useKeyboardShortcuts.ts

@@ -1,6 +1,7 @@
 import React from 'react';
 import { isTerminalEventTarget } from '@/lib/terminalFocus';
 import { useSessionUIStore } from '@/sync/session-ui-store';
+import { closeSessionTabAndActivateNeighbour } from '@/lib/sessionTabs';
 import { useSelectionStore } from '@/sync/selection-store';
 import * as sessionActions from '@/sync/session-actions';
 import { normalizeContextPanelDirectoryKey, useUIStore } from '@/stores/useUIStore';
@@ -299,6 +300,14 @@ export const useKeyboardShortcuts = () => {
         return;
       }
 
+      if (!isVSCodeRuntime() && eventMatchesShortcut(e, combo('close_session_tab'))) {
+        e.preventDefault();
+        if (currentSessionId) {
+          closeSessionTabAndActivateNeighbour(currentSessionId);
+        }
+        return;
+      }
+
       const matchedNewSessionShortcut = eventMatchesShortcut(e, combo('new_chat'));
       const matchedWorktreeShortcut = eventMatchesShortcut(e, combo('new_chat_worktree'));