grid: fix fade animation cleanup crashing FlowLayout.clear

The previous deleteLater on the QPropertyAnimation left a dangling
self._fade_anim reference to a dead C++ object. When the next
search called FlowLayout.clear(), calling .stop() on the dead
animation threw RuntimeError and aborted widget cleanup, leaving
stale thumbnails in the grid.

Now the finished callback nulls self._fade_anim before scheduling
deletion, so clear() never touches a dead object.

booru_viewer/gui/grid.py

@@ -111,10 +111,16 @@ class ThumbnailWidget(QWidget):
         anim.setStartValue(0.0)
         anim.setEndValue(1.0)
         anim.setEasingCurve(QEasingCurve.Type.OutCubic)
-        anim.finished.connect(anim.deleteLater)
+        anim.finished.connect(lambda: self._on_fade_done(anim))
         self._fade_anim = anim
         anim.start()
 
+    def _on_fade_done(self, anim: QPropertyAnimation) -> None:
+        """Clear the reference then schedule deletion."""
+        if self._fade_anim is anim:
+            self._fade_anim = None
+        anim.deleteLater()
+
     def set_selected(self, selected: bool) -> None:
         self._selected = selected
         self.update()