PDF links: extract pdf_links.{cpp,h} shared helper

Move the PDF cross-reference hyperlink logic out of ProjectPrintWindow
into a standalone translation unit so it can be reused (the CLI PDF
export will call it next):

- injectCrossRefLinks(): emits the URI link annotations for a diagram's
  cross-references and folio reports. The scene-to-page mapping is passed
  in as a PageGeometry (transform + devToPdf + source-rect lookup) so each
  caller supplies its own correct geometry, rather than the helper assuming
  a QPrinter.
- convertUriToGoTo(): the PDF post-processor, moved verbatim.

ProjectPrintWindow stays a pure caller: it builds its PageGeometry from the
printer page layout exactly as before and calls the helper. No behavioural
change to GUI PDF export; no class-structure changes.

Per review guidance on #483.

sources/pdf_links.h

@@ -0,0 +1,79 @@
+/*
+	Copyright 2006-2025 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef PDF_LINKS_H
+#define PDF_LINKS_H
+
+#include <QMap>
+#include <QPointF>
+#include <QRectF>
+#include <QString>
+#include <QTransform>
+#include <functional>
+
+class QPdfEngine;
+class Diagram;
+
+/**
+	Shared helper that turns a project's cross-references and folio reports
+	into clickable internal hyperlinks in a Qt-generated PDF.  Used by both the
+	GUI print path (ProjectPrintWindow) and the headless CLI export, each of
+	which builds its own page geometry and passes it in — this code never
+	computes the scene-to-page mapping itself.
+*/
+namespace PdfLinks {
+
+	/**
+		Geometry mapping for one rendered PDF page.  Each caller builds this
+		from its OWN page setup (printer page layout vs QPdfWriter), since the
+		device-pixel and point conversions differ between them.
+	*/
+	struct PageGeometry {
+		/// scene coordinates -> device pixels (the same "fit" render() applied)
+		QTransform sceneToDevice;
+		/// device paint rectangle, in pixels (the page area)
+		QRectF target;
+		/// links whose rectangle falls outside this are dropped
+		QRectF pageBounds;
+		/// device pixels -> PDF points (replicates the engine's page matrix)
+		std::function<QPointF(const QPointF &)> devToPdf;
+		/// a diagram -> its source rectangle in scene pixels (for /FitR framing)
+		std::function<QRectF(Diagram *)> sourceRectOf;
+	};
+
+	/**
+		Inject clickable cross-reference / folio-report hyperlinks for @p diagram
+		into the current page of @p engine.  Each link is emitted as a URI
+		annotation encoding the target page and a /FitR rectangle;
+		convertUriToGoTo() then rewrites those into native internal GoTo actions.
+	*/
+	void injectCrossRefLinks(QPdfEngine *engine, Diagram *diagram,
+							 const PageGeometry &geom,
+							 const QMap<Diagram *, int> &pageMap,
+							 const QString &outputFileName);
+
+	/**
+		Post-process a Qt-generated PDF file: rewrite every "/S /URI" link
+		annotation into a native internal "/S /GoTo" action (page + /FitR or
+		/Fit destination) and rebuild the xref table.  No-op if the file has no
+		such annotations.
+	*/
+	void convertUriToGoTo(const QString &pdfPath);
+
+}
+
+#endif // PDF_LINKS_H