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/lib/shortcuts.ts

@@ -239,6 +239,13 @@ const SHORTCUT_ACTIONS: ReadonlyArray<ShortcutAction> = [
     description: 'Create a new worktree and open a draft in it',
     customizable: true,
   },
+  {
+    id: 'close_session_tab',
+    defaultCombo: 'alt+w',
+    label: 'Close session tab',
+    description: 'Close the active session tab in the header (the session itself stays)',
+    customizable: true,
+  },
   {
     id: 'new_mini_chat',
     defaultCombo: 'mod+alt+n',