fix(ui): tách hàng Cài đặt khỏi nhóm menu ngay trên nó

Thanh rail đặt setSpacing(0) nên nút Cài đặt dán sát Dashboard/Giám sát, ba
dòng đọc thành một khối. Thêm hằng _NAV_SETTINGS_GAP trong rail_metrics và
xin khe bằng nvl.addSpacing() — margin trong QSS vẽ BÊN TRONG hộp widget, mà
nút này bị _rebuild_nav ghim đúng chiều cao một dòng menu nên margin không
mua được pixel nào.

Rút gọn lại comment ở _rebuild_nav cho khớp.

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

presentation/shell/nav_rail.py

@@ -251,24 +251,17 @@ class NavRailMixin:
                 blocked = tree.blockSignals(True)
                 tree.addTopLevelItem(it)
                 tree.blockSignals(blocked)
-            # Both destination lists are exactly as tall as their rows; the
-            # stretch in between belongs to RECENTS.
-            #
-            # The frame, and nothing else. A flat ``+ 8`` here used to leave 6px
-            # of dead space under the last row of each list, and because the
-            # Settings button sits DIRECTLY under nav_bottom (nvl has no
-            # spacing), that space landed between Giám sát and Settings only —
-            # so three rows that read as one list were spaced 18/26px. Padding
-            # a row is the item delegate's job; this is the frame's.
+            # Rows plus frame, nothing else: the flat ``+ 8`` this replaces
+            # left 6px of dead space under the last row, and since Settings
+            # sits directly under nav_bottom it fell between Giám sát and
+            # Settings alone — 18/26px for rows that read as one list.
             row_h = 0
             for tree in (self.nav, self.nav_bottom):
                 n = tree.topLevelItemCount()
                 row_h = tree.sizeHintForRow(0) if n else row_h
                 tree.setFixedHeight(n * row_h + 2 * tree.frameWidth())
-            # Settings is one more row of the same list, so it gets the rows'
-            # own height rather than a second set of paddings guessed to match
-            # it — the only way the three stay evenly spaced when the font (and
-            # with it ``sizeHintForRow``) is not the one this was tuned on.
+            # Settings is one more row of the list, so it takes the rows' own
+            # height instead of paddings guessed to match it.
             if row_h and hasattr(self, "_nav_settings_btn"):
                 self._nav_settings_btn.setFixedHeight(row_h)
             if keep: