Fix bugtracker #340: small PDF text drawn too bold in Adobe Acrobat

Qt 6 declares the PDF/X namespace (pdfxid) in the XMP metadata of every
PDF it writes, even when the file is not PDF/X. Adobe Acrobat then draws
small text too bold at some zoom levels. Qt only needs the declaration
for PDF/X-4 output, which QElectroTech never asks for.

Blank the declaration out after export, in place with spaces so no
offsets move, in both the export window and the --export-pdf path.
Diagnosis and the Acrobat testing by Alf, bugtracker #340.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/cli_export.cpp

@@ -227,6 +227,7 @@ int exportPdf(QETProject &project, const QString &output,
 	// Rewrite the URI link annotations into native internal GoTo actions, so
 	// the cross-references jump inside the document in any PDF viewer.
 	PdfLinks::convertUriToGoTo(output);
+	PdfLinks::removeUnusedPdfxNamespace(output);
 
 	out << "Exported " << diagrams.size() << " page(s) -> " << output << "\n";
 	return 0;