Add read-only enforcement, preview, import/export to custom SQL reports

Refs discussion #886: a saved-report manager built on custom SQL and
nomenclature.json. Most of what was asked for already existed --
Projet > Exporter au format CSV already builds/saves/reuses named
SELECT queries via ElementQueryWidget and nomenclature.json, and
Projet > Ajouter une nomenclature already inserts one into a folio.
This fills the three real gaps.

- projectDataBase::isReadOnlySelect() rejects anything that isn't a
  single SELECT/WITH statement. Checked in newQuery() itself, the one
  choke point every query path already goes through -- including a
  query loaded from a saved nomenclature/summary table's <query>
  element on project open, not just the dialog's own custom-SQL box.
  ElementQueryWidget shows the same check live as you type, and
  BOMExportDialog surfaces it before running or exporting anything.
- BOMExportDialog gains a Preview button + table (QSqlQueryModel),
  so a report can be checked on screen before committing to a CSV file.
- ElementQueryWidget gains Importer.../Exporter... buttons that
  read/write nomenclature.json's saved reports as a JSON file, so a
  report can be handed to a colleague or another install. Import asks
  before overwriting a locally-saved report of the same name.

Verified: Qt 6.10.2, builds clean, ctest 8/8. Drove the real dialog
through Xvfb: typed "DROP TABLE element" into the custom-SQL box and
got the inline warning immediately, then confirmed Preview also
refuses it with a "Requête refusée" dialog rather than running it.
Preview against the real default query returned live column headers
and a row. Export opens a save dialog without crashing.

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

sources/dataBase/ui/elementquerywidget.h

@@ -59,6 +59,9 @@ class ElementQueryWidget : public QWidget
 		void on_m_load_pb_clicked();
 		void on_m_save_current_conf_pb_clicked();
 		void on_m_save_name_le_textChanged(const QString &arg1);
+		void on_m_import_reports_pb_clicked();
+		void on_m_export_reports_pb_clicked();
+		void checkQueryValidity();
 		void on_m_choosen_list_currentItemChanged(QListWidgetItem *current, QListWidgetItem *previous);
 		void on_m_var_list_itemDoubleClicked(QListWidgetItem *item);
 		void on_m_choosen_list_itemDoubleClicked(QListWidgetItem *item);