Simplify Models panel and dialog layout

Models panel: replace the manual resizeEvent column-sizing hack with
QHeaderView Stretch/Fixed modes, re-applied via sectionCountChanged so
they survive the model rebuilds that QHeaderView resets them on.

Dialog: only wrap the body in a QScrollArea when scrollable, mirroring
Panel. The scroll area caps its sizeHint at 36x24 cells, which turned
wide fixed-size dialog content into spurious scrollbars.

Add Model dialog: reserve a stable, font-metrics-measured height for the
hover description so longer text never reflows the buttons; regroup the
buttons into LOCAL / CLOUD / TOOLS.

Buttons: move the trailing-separator decision out of makeButtonGroup
into a new addButtonGroups row builder, so the last group in a row
never draws a dangling divider.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/bonsaiviewer/components/Buttons.h

@@ -23,6 +23,7 @@
 
 #include <QList>
 
+class QBoxLayout;
 class QToolButton;
 class QWidget;
 
@@ -35,9 +36,13 @@ QToolButton* makeButton(const QString& text,
 QWidget* makeButtonGroup(const QString& title,
                          const QList<QToolButton*>& buttons,
                          QWidget* parent,
-                         bool trailing_separator = true,
                          int vertical_spacing = 4);
 
+// Adds button groups to a ribbon row, drawing a vertical divider between
+// adjacent groups but never after the last one. Centralising the decision
+// here means a row can't end up with a dangling trailing separator.
+void addButtonGroups(QBoxLayout* row, const QList<QWidget*>& groups);
+
 } // namespace bonsaiviewer::components::buttons
 
 #endif