Add headless command-line export (PDF/PNG/SVG)

Implements the long-requested batch/headless export
(bugtracker #171, GitHub #309): render a project's diagrams to files
without opening the GUI.

  qelectrotech --export-pdf <project.qet> <output.pdf>   one multi-page PDF
  qelectrotech --export-png <project.qet> <output_dir>   one PNG per diagram
  qelectrotech --export-svg <project.qet> <output_dir>   one SVG per diagram

main.cpp detects an export request before SingleApplication is created (so the
arguments are not forwarded to a running instance), spins up a plain
QApplication for rendering, and exits with the export's status code.

Rendering reuses Diagram::render() over
BorderTitleBlock::borderAndTitleBlockRect(), the same geometry the GUI
print/export path uses, so output matches the editor. Image files are named
NN_Title.<ext>.

New files: sources/cli_export.{h,cpp}, registered in
cmake/qet_compilation_vars.cmake.

sources/cli_export.h

@@ -0,0 +1,56 @@
+/*
+	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 CLI_EXPORT_H
+#define CLI_EXPORT_H
+
+#include <QStringList>
+
+/**
+	@brief Headless command-line export.
+
+	Implements the long-requested batch/headless export
+	(qelectrotech.org bugtracker #171, GitHub #309): render a project's
+	diagrams to files without opening the GUI.
+
+	Detected and handled in main() before the GUI is created.
+*/
+namespace CLIExport {
+
+	/**
+		@brief True if @p args request a CLI export
+		(i.e. contain one of the export options).
+	*/
+	bool isExportRequest(const QStringList &args);
+
+	/**
+		@brief Run the CLI export described by @p args.
+		@return process exit code (0 on success).
+
+		Usage:
+		  qelectrotech --export-pdf  <project.qet> <output.pdf>
+		  qelectrotech --export-png  <project.qet> <output_dir>
+		  qelectrotech --export-svg  <project.qet> <output_dir>
+
+		PDF: one multi-page document (one diagram per page).
+		PNG/SVG: one file per diagram, named <output_dir>/<NN>_<title>.<ext>.
+	*/
+	int run(const QStringList &args);
+
+}
+
+#endif // CLI_EXPORT_H