refactor: pen_editor_screen._centerPage uses shared centerOffset

Wires the now-tested viewport_fit.centerOffset into the live editor, replacing
the inline ad-hoc arithmetic. Behavior-identical (same centering at scale 1) —
makes the pure core load-bearing and kills the duplicate math.

flutter analyze lib/editor clean; 228/228 tests pass (no behavior change).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/canvas/pen_editor_screen.dart

@@ -15,6 +15,7 @@ import '../engine/undo_stack.dart';
 import '../input/diagnostic_logger.dart';
 import '../input/pen_config.dart';
 import '../input/pen_input_service.dart';
+import '../layout/viewport_fit.dart';
 import '../persistence/editor_repository.dart';
 import '../persistence/save_scheduler.dart';
 import '../ui/pen_settings_page.dart';
@@ -381,11 +382,12 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     showPenSettingsSheet(context, config);
   }
 
-  /// Centre [pageSize] within [viewport] via the shared transform.
+  /// Centre [pageSize] within [viewport] via the shared transform. Uses the
+  /// shared, unit-tested [centerOffset] (pageSize is already fit to the viewport
+  /// at scale 1, so we center at scale 1).
   void _centerPage(Size viewport, Size pageSize) {
-    final tx = (viewport.width - pageSize.width) / 2;
+    final o = centerOffset(pageSize, viewport, 1.0);
-    final ty = (viewport.height - pageSize.height) / 2;
+    _transform.value = Matrix4.identity()..setTranslationRaw(o.dx, o.dy, 0);
-    _transform.value = Matrix4.identity()..setTranslationRaw(tx, ty, 0);
   }
 
   @override