Add diagram background color picker with adaptive border/titleblock

Replace the white/grey toggle (m_grey_background) with a full color
picker widget (DiagramBgColorToolButton) in the Affichage toolbar,
matching the existing ConductorColorToolButton UX:

- Preset colors: White, Off-white, Light grey, Grey, Dark grey, Black
- Recently used colors section
- "Autre couleur..." opens QColorDialog for any custom color
- "Couleur système" restores the default dark-mode inverted background

New features:
- Diagram::m_custom_background_color flag: when the user picks a
  custom color, PaletteGraphicsView skips lightness inversion so the
  chosen color is displayed as-is
- Border and titleblock text/lines automatically switch between black
  and white based on Diagram::background_color.lightness(), so a dark
  background always shows a visible light border and titleblock content
- "Couleur système" restores Qt::white + re-enables inversion

Files changed:
- New: sources/ui/diagrambgcolorbutton.h/.cpp
- sources/diagram.h/.cpp: added static m_custom_background_color flag
- sources/palettegraphicsview.cpp: skip inversion when custom bg active
- sources/bordertitleblock.cpp: adaptive border pen color
- sources/titleblocktemplate.cpp: adaptive ink color for cell borders/text
- sources/qetdiagrameditor.h/.cpp: replace toggle with new widget
- cmake/qet_compilation_vars.cmake: register new source files

sources/qetdiagrameditor.cpp

@@ -51,6 +51,7 @@
 #include "shortcutmanager.h"
 #include "ui/bomexportdialog.h"
 #include "ui/conductorcolortoolbutton.h"
+#include "ui/diagrambgcolorbutton.h"
 #include "ui/jumptoelementdialog.h"
 #include "ui/diagrampropertieseditordockwidget.h"
 #include "ui/backupdialog.h"
@@ -413,15 +414,8 @@ void QETDiagramEditor::setUpActions()
 			pv->project()->setAutoBreakConductor(abc);
 	});
 
-		//Switch background color
-	m_grey_background = new QAction   (QET::Icons::DiagramBg, tr("Couleur de fond blanc/gris","Tool tip of white/grey background button"), this);
-	m_grey_background -> setStatusTip (tr("Affiche la couleur de fond du folio en blanc ou en gris", "Status tip of white/grey background button"));
-	m_grey_background -> setCheckable (true);
-	connect (m_grey_background, &QAction::triggered, [this](bool checked) {
-		Diagram::background_color = checked ? Qt::darkGray : Qt::white;
-		if (this->currentDiagramView() &&  this->currentDiagramView()->diagram())
-			this->currentDiagramView()->diagram()->update();
-	});
+		//Diagram background color picker
+	m_background_color_button = new DiagramBgColorToolButton(this, this);
 
 		//Draw or not the background grid
 	m_draw_grid = new QAction ( QET::Icons::Grid, tr("Afficher la grille"), this);
@@ -912,7 +906,7 @@ void QETDiagramEditor::setUpToolBar()
 	view_tool_bar -> addSeparator();
 	view_tool_bar -> addAction(m_draw_grid);
 	view_tool_bar -> addAction(m_draw_guides);
-	view_tool_bar -> addAction (m_grey_background);
+	view_tool_bar -> addWidget(m_background_color_button);
 	view_tool_bar -> addSeparator();
 	view_tool_bar -> addActions(m_zoom_action_toolBar);
 
@@ -1063,7 +1057,7 @@ void QETDiagramEditor::setUpMenu()
 	menu_affichage -> addSeparator();
 	menu_affichage -> addAction(m_draw_grid);
 	menu_affichage -> addAction(m_draw_guides);
-	menu_affichage -> addAction(m_grey_background);
+	menu_affichage -> addMenu(m_background_color_button->menu());
 	menu_affichage -> addSeparator();
 	menu_affichage -> addActions(m_zoom_actions_group.actions());
 
@@ -1880,7 +1874,7 @@ void QETDiagramEditor::slot_updateActions()
 	m_select_actions_group.         setEnabled(opened_diagram);
 	m_add_item_actions_group.       setEnabled(editable_project);
 	m_row_column_actions_group.     setEnabled(editable_project);
-	m_grey_background->             setEnabled(opened_diagram);
+	m_background_color_button->    setEnabled(opened_diagram);
 	m_draw_grid->                   setEnabled(opened_diagram);
 	m_draw_guides->                 setEnabled(opened_diagram);