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.ui

@@ -388,6 +388,26 @@
         </property>
        </widget>
       </item>
+      <item row="2" column="0">
+       <widget class="QPushButton" name="m_import_reports_pb">
+        <property name="toolTip">
+         <string>Importer des rapports depuis un fichier</string>
+        </property>
+        <property name="text">
+         <string>Importer...</string>
+        </property>
+       </widget>
+      </item>
+      <item row="2" column="1">
+       <widget class="QPushButton" name="m_export_reports_pb">
+        <property name="toolTip">
+         <string>Exporter tous les rapports enregistrés vers un fichier</string>
+        </property>
+        <property name="text">
+         <string>Exporter...</string>
+        </property>
+       </widget>
+      </item>
      </layout>
     </widget>
    </item>
@@ -429,6 +449,19 @@
      </item>
     </layout>
    </item>
+   <item>
+    <widget class="QLabel" name="m_query_warning_label">
+     <property name="styleSheet">
+      <string notr="true">color: red;</string>
+     </property>
+     <property name="text">
+      <string/>
+     </property>
+     <property name="wordWrap">
+      <bool>true</bool>
+     </property>
+    </widget>
+   </item>
   </layout>
  </widget>
  <resources>