fix(ui): prevent nav rail from collapsing to zero width

- Lower _NAV_MIN_WIDTH to 54px (collapsed icon-only width) so the
  splitter never allows a state where the toggle button disappears
- Auto-snap to collapsed mode when user drags below threshold instead
  of leaving the rail stuck at an unusable width

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

presentation/shell/rail_metrics.py

@@ -15,7 +15,7 @@ _NAV_EXPANDED_WIDTH = 150
 _NAV_COLLAPSED_WIDTH = 54
 _NAV_ROW_INSET = 8
 _NAV_ROW_GAP = 6
-_NAV_MIN_WIDTH = 132
+_NAV_MIN_WIDTH = 54
 _NAV_MAX_SHARE = 0.22
 _NAV_MAX_CEILING = 360