Fix macOS file picker not appearing and Windows drag-and-drop freeze

- macOS: remove window minimize before FilePicker dialog — the
  NSOpenPanel handles z-order natively; minimizing caused the dialog
  to appear hidden in the dock (returns null, user sees nothing).
  The underlying window ordering issue was already fixed in the
  MainFlutterWindow.swift refactor.
- Windows: delay _handleDrop by one frame after OLE DnD completes so
  Flutter can finish the drag message loop before pushing the import
  dialog, preventing the grey/frozen window state.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Fenron-dev

lib/core/file_picker_helper.dart

@@ -19,8 +19,7 @@ class FilePickerHelper {
 
   static bool get _isDesktop =>
       Platform.isLinux || Platform.isWindows || Platform.isMacOS;
-  static bool get _shouldMinimizeDesktopWindow =>
-      Platform.isLinux || Platform.isMacOS;
+  static bool get _shouldMinimizeDesktopWindow => Platform.isLinux;
 
   static bool _effectiveLockParentWindow(bool lockParentWindow) =>
       lockParentWindow || Platform.isWindows;

lib/ui/widgets/drop_zone.dart

@@ -32,7 +32,12 @@ class _DropZoneOverlayState extends ConsumerState<DropZoneOverlay> {
       onDragExited: (_) => setState(() => _isDragging = false),
       onDragDone: (detail) {
         setState(() => _isDragging = false);
-        _handleDrop(detail.files);
+        // Delay by one frame so Flutter can finish the OLE DnD message loop
+        // on Windows before the dialog is pushed — without this the window
+        // stays grey/frozen after the drop.
+        WidgetsBinding.instance.addPostFrameCallback((_) {
+          _handleDrop(detail.files);
+        });
       },
       child: Stack(
         fit: StackFit.expand,