Fix Project properties > OK marking the project modified with no edit

Pressing OK in Projet > Propriétés du projet without changing anything
marked the project modified and rewrote its defaults for new folios.
Besides the Page Num placeholder (fixed separately), four causes:

- Title block custom fields: since #271 the dialog offers the
  template's variables for filling in; those left empty came back as
  part of the context. Drop the ones not in the stored context.
- DiagramContext::remove() left the key's show flag behind, so a
  context with a key removed never equalled one without it.
- Guides: applyConf() marked the project modified whether or not the
  guides changed. Compare them like every other setting.
- Wire dash size and cross-reference offsets: the spin boxes cannot
  show the stored default (dash size 1 under a minimum of 2; offset 0
  under a minimum of 40 shown as "Default"), so the clamped value came
  back. Keep the stored value unless the box shows something else, as
  #1204 did for the wire panel.

Checked in the GUI (with the Page Num fix as well) on grafcet,
schema_indus and Projet_vierge: OK with no edit then Ctrl+S saves the
same file as Ctrl+S alone; master writes dash-size 1 -> 2, a
cross-reference offset 0 -> 40, and on Projet_vierge 22 empty custom
fields. New test for DiagramContext::remove() fails without the fix.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

sources/ui/conductorpropertieswidget.cpp

@@ -126,6 +126,12 @@ ConductorProperties ConductorPropertiesWidget::properties() const
 	properties_.m_bicolor               = ui->m_color_2_gb->isChecked();
 	properties_.m_color_2               = ui->m_color_2_kpb->color();
 	properties_.m_dash_size             = ui->m_dash_size_sb->value();
+		//The box starts at 2, so the default size 1 shows as 2: keep the
+		//stored size unless the box shows something else.
+	if (properties_.m_dash_size == qBound(ui->m_dash_size_sb->minimum(),
+										  m_properties.m_dash_size,
+										  ui->m_dash_size_sb->maximum()))
+		properties_.m_dash_size = m_properties.m_dash_size;
 	properties_.style                   = ui -> m_line_style_cb->itemData(ui->m_line_style_cb->currentIndex()).value<QPen>().style();
 	properties_.m_formula               = ui->m_formula_le->text();
 	properties_.text                    = ui -> m_text_le -> text();