fix: image_canvas correctness + RAM fixes (v1.3.0)

- _imageSize reset on image switch (wrong swap threshold on nav)
- downsample caps both dims (tall images no longer full-res decode)
- single _readImageSize instead of leaking loadingBuilder listeners
- evict old downsample provider on full-res swap
- remove imageCache.clear() that dropped live image (gallery flash)

lib/services/image_manager.dart

@@ -69,8 +69,6 @@ class ImageManager extends ChangeNotifier {
     }
     if (_currentIndex < 0) _currentIndex = 0;
 
-    // Eski klasörün decode edilmiş görsellerini bırak; canlı görsel korunur
-    PaintingBinding.instance.imageCache.clear();
     notifyListeners();
   }