Altium Schematic Import: Fix duplicate sheet creation in hierarchical projects

When importing Altium projects, sheets were sometimes imported as both top-level
sheets and hierarchical subsheets. This happened because the project file lists
sheets in arbitrary order. If a parent sheet was processed before its children,
the children would be loaded as subsheets during hierarchy descent. Later, when
those same files appeared in the project file, duplicate sheets were created.

The importer now checks whether a file has already been loaded as a subsheet
before creating a new top-level sheet entry. The check includes a case-insensitive
fallback search since Altium uses inconsistent casing in its .SchDoc extension.

Sheet filenames were also being set to the original Altium paths instead of
KiCad project-relative paths. This caused path mismatches in the hierarchy
search and left references to non-existent Altium paths in the saved schematic.
Filenames now use the KiCad project directory with the .kicad_sch extension.

Hierarchical labels on top-level sheets are converted to global labels during
import since top-level sheets have no parent sheet to connect them to.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21909

(cherry picked from commit f00dc1befd)

eeschema/sch_io/altium/sch_io_altium.cpp

@@ -595,6 +595,37 @@ SCH_SHEET* SCH_IO_ALTIUM::LoadSchematicFile( const wxString& aFileName, SCHEMATI
     else
         ParseAltiumSch( aFileName );
 
+    // Convert hierarchical labels to global labels on top-level sheets.
+    // Top-level sheets have no parent, so hierarchical labels don't make sense.
+    if( aFileName.empty() )
+    {
+        for( SCH_ITEM* item : rootScreen->Items().OfType( SCH_SHEET_T ) )
+        {
+            SCH_SHEET* sheet = static_cast<SCH_SHEET*>( item );
+            SCH_SCREEN* screen = sheet->GetScreen();
+
+            if( !screen )
+                continue;
+
+            std::vector<SCH_HIERLABEL*> hierLabels;
+
+            for( SCH_ITEM* labelItem : screen->Items().OfType( SCH_HIER_LABEL_T ) )
+                hierLabels.push_back( static_cast<SCH_HIERLABEL*>( labelItem ) );
+
+            for( SCH_HIERLABEL* hierLabel : hierLabels )
+            {
+                SCH_GLOBALLABEL* globalLabel = new SCH_GLOBALLABEL( hierLabel->GetPosition(),
+                                                                     hierLabel->GetText() );
+                globalLabel->SetShape( hierLabel->GetShape() );
+                globalLabel->SetSpinStyle( hierLabel->GetSpinStyle() );
+
+                screen->Remove( hierLabel );
+                screen->Append( globalLabel );
+                delete hierLabel;
+            }
+        }
+    }
+
     if( m_reporter )
     {
         for( auto& [msg, severity] : m_errorMessages )