Edit increment and preview the next number in the auto-numbering dock

Bug #331: "Il serait intéressant de pouvoir directement dans la fenêtre
'Sélection numérotation auto' modifier la valeur d'incrément et visualiser
la prochaine numérotation qui sera appliquée. Ceci sans être obligé
d'ouvrir la page de configuration."

The dock (AutoNumberingDockWidget) already let you see and edit a rule's
*current* value inline (added in 52c8ef6b4/031710b5f/ee4ba82d2). The
increment itself, and any preview of where the numbering is headed, was
reachable only through Configurer -> the full project-properties dialog.

Two new widgets per row (conductor/element/folio):

- An increment spin box, read from and written to the same NumerotationContext
  field NumPartEditorW's increase_spinBox already edits in the full dialog --
  same data, second place to reach it.
- A read-only next-value field, computed via
  NumerotationContextCommands::next() -- the identical engine the "Suivant"
  button in the full dialog already uses to step a whole context. Reusing it
  rather than reimplementing the arithmetic means wrap-and-carry between parts
  comes out identical to what actually happens when the number is next
  consumed, and zero-padding matches real rendering
  (NumerotationContext::formatValue(), mirroring
  autonum::setSequentialToList()'s padding rule by hand since that function is
  local to assignvariables.cpp).

NumerotationContext gains replaceIncrease(index, increase), a sibling to the
existing replaceValue() that touches only the increment field.

Every refresh call site in the file (13 of them) previously refreshed just the
value field; they now go through a new refreshRow(category), which refreshes
value + increment + next-value-preview together via a small per-row widget
bundle (rowFor()). This also let resetAutoNum()'s three-way switch collapse to
one line, and refreshValueFields()'s three near-identical blocks collapse to a
loop -- both existing before this change, not new here.

Verified live under Xvfb: created an element numbering rule "K" (Chiffre 1,
value 1, increment 1) via the full dialog, confirmed the dock showed
Valeur=1/Incrément=1/Suivant=2. Changed the dock's own Incrément to 3 --
Suivant updated live to 4, no dialog needed. Changed Valeur to 10 -- Suivant
became 13. Reopened the full configuration dialog and confirmed it read back
the same value_field=10/increase_spinBox=3, i.e. the round trip through
replaceIncrease()/storeContext() does not disturb type, initial value, modulus
or format.

Builds clean, CMake/Ninja Release, Qt 5.15, 820/820, no new warnings.

Fixes: https://qelectrotech.org/bugtracker/view.php?id=331

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

sources/autoNum/numerotationcontext.h

@@ -54,6 +54,11 @@ class NumerotationContext
 	QDomElement toXml(QDomDocument &, const QString&);
 	void fromXml(QDomElement &);
 	void replaceValue(int, QString);
+	void replaceIncrease(int, int);
+		/// Zero-pad a part's value the same way the real numbering engine
+		/// does (autonum::setSequentialToList in assignvariables.cpp), so a
+		/// UI preview of a part's value matches what actually gets rendered.
+	static QString formatValue(const QStringList &item);
 
 	private:
 	QStringList content_;