Stop the rail destinations sliding to the middle when it collapses

The scroll body holds [nav, RECENTS header, RECENTS], and RECENTS carries the
only stretch factor. Collapsing hides it, which leaves a fixed-height nav tree
(app.py sets one per row count) and nothing that can expand — so the box layout
centred what was left and the destinations dropped 297px down a 1048px window,
with the same gap opening below them.

A stretch spacer at the foot of the body takes the slack instead. It takes none
of it while RECENTS is visible, because RECENTS carries stretch 1 against its 0,
so the open rail is byte-for-byte what it was.

This is what "thu gọn menu lại ra giữa" meant, reported three times. I read it
as horizontal each time and measured x — which was 4px and correct throughout.
The report was about the vertical axis and I never checked it.

check_rail_align now measures the gap between the ＋ button and the first
destination in both states: 6px/6px now, 6px/223px with the spacer removed.

15/15 checkers pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app.py

@@ -285,6 +285,12 @@ class MainWindow(QMainWindow):
         self.nav_recents = self._new_nav_tree("navRecents")
         self.nav_recents.itemClicked.connect(self._on_rail_recent)
         sv.addWidget(self.nav_recents, 1)
+        # Collapsing hides RECENTS, and with it the only item carrying a stretch
+        # factor. A box layout with nothing left to expand centres what remains,
+        # so the destinations dropped ~300px down the rail — "thu gọn menu lại
+        # ra giữa". This spacer takes the slack instead, and takes none of it
+        # while RECENTS is visible (stretch 0 against its 1).
+        sv.addStretch(0)
         self._nav_scroll.setWidget(scroll_body)
         nvl.addWidget(self._nav_scroll, 1)
         # Bottom-pinned group: the places you visit occasionally, kept out of the