add window resize handles for frameless mode

Without OS decorations there are no resize handles. Added 6px
edge and 12px corner invisible hit zones around the window
perimeter via ClideResizeBorder. Each zone changes the cursor
and calls gtk_window_begin_resize_drag via the clide/window
method channel with a GdkWindowEdge direction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/window_controls.dart

@@ -3,6 +3,8 @@ import 'package:flutter/services.dart';
 
 enum ChromeStyle { seam, prompt, inline }
 
+enum ResizeEdge { topLeft, top, topRight, left, right, bottomLeft, bottom, bottomRight }
+
 class WindowControls extends ChangeNotifier {
   static const _channel = MethodChannel('clide/window');
 
@@ -15,6 +17,14 @@ class WindowControls extends ChangeNotifier {
     notifyListeners();
   }
 
+  Future<void> startResize(ResizeEdge edge) async {
+    try {
+      await _channel.invokeMethod('startResize', edge.index);
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
   Future<void> startDrag() async {
     try {
       await _channel.invokeMethod('startDrag');