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

cmake/qet_compilation_vars.cmake

@@ -707,6 +707,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/ui/conductorpropertiesdialog.h
   ${QET_DIR}/sources/ui/conductorcolortoolbutton.cpp
   ${QET_DIR}/sources/ui/conductorcolortoolbutton.h
+  ${QET_DIR}/sources/ui/diagrambgcolorbutton.cpp
+  ${QET_DIR}/sources/ui/diagrambgcolorbutton.h
   ${QET_DIR}/sources/ui/conductorpropertieswidget.cpp
   ${QET_DIR}/sources/ui/conductorpropertieswidget.h
   ${QET_DIR}/sources/ui/configsaveloaderwidget.cpp