Fix #1113: custom title-block variables in Find/Replace and unset placeholders

Two problems with a custom variable newly added to a title-block template:

1. Find/Replace > Folio > Custom showed an empty table, so the user had to
   know and type each variable name. It now lists every custom variable the
   folios already carry plus those their templates use, with empty values.
   An empty value now means "leave unchanged", like every field of the main
   tab; only the variables actually filled in are written to the folios.

2. The title block showed the variable's own name ("%doc-type") until the
   folio's properties were opened, because interpreteVariables() only
   replaces names present in the context. Placeholders in the template text
   that no key resolves now render blank, as auto-added unset ones already
   did since #973. Only the template's own text is considered, so a value
   that contains "%something" is never touched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/titleblocktemplate.cpp

@@ -27,6 +27,7 @@
 
 #include <QRegularExpression>
 #include <QRegularExpressionMatch>
+#include <algorithm>
 /**
 	@brief TitleBlockTemplate::TitleBlockTemplate
 	Constructor
@@ -1818,14 +1819,46 @@ QString TitleBlockTemplate::interpreteVariables(
 		const QString &string,
 		const DiagramContext &diagram_context) const
 {
+	// A variable nobody has given a value to yet -- e.g. one just added to
+	// the template, which is not in the folio's Custom tab until its
+	// properties are opened (#1113) -- must render blank rather than as its
+	// own name, the same as an auto-added but unset one already does (#973).
+	// Collect those from the template text before substituting, so a value
+	// that happens to contain "%something" is never touched.
+	// A bare "%name" is unset only if no key is a prefix of it, because the
+	// substitution below replaces "%key" wherever it appears.
+	static const QRegularExpression rx(
+		QStringLiteral("%\\{([a-z0-9_-]+)\\}|%([a-z0-9_-]+)"));
+	const QStringList keys =
+		diagram_context.keys(DiagramContext::DecreasingLength);
+	QStringList unset;
+	auto it = rx.globalMatch(string);
+	while (it.hasNext()) {
+		const QRegularExpressionMatch m = it.next();
+		const QString name = m.captured(1).isEmpty()
+				? m.captured(2) : m.captured(1);
+		bool known = diagram_context.contains(name);
+		if (!known && m.captured(1).isEmpty()) {
+			for (const QString &key : keys) {
+				if (name.startsWith(key)) { known = true; break; }
+			}
+		}
+		if (!known) unset << m.captured(0);
+	}
+
 	QString interpreted_string = string;
-	foreach (QString key,
-		 diagram_context.keys(DiagramContext::DecreasingLength)) {
+	foreach (QString key, keys) {
 		interpreted_string.replace("%{" % key % "}",
 					   diagram_context[key].toString());
 		interpreted_string.replace("%" % key,
 					   diagram_context[key].toString());
 	}
+	std::sort(unset.begin(), unset.end(),
+		  [](const QString &a, const QString &b) {
+			return a.length() > b.length();
+		  });
+	for (const QString &placeholder : unset)
+		interpreted_string.remove(placeholder);
 	return(interpreted_string);
 }
 
@@ -1834,7 +1867,7 @@ QString TitleBlockTemplate::interpreteVariables(
 	Get list of variables
 	@return The list of string with variables
 */
-QStringList TitleBlockTemplate::listOfVariables()
+QStringList TitleBlockTemplate::listOfVariables() const
 {
 	QStringList list;
 	// Match both the braced "%{name}" form and the bare "%name" form