- ThumbnailWidget detects clicks outside the pixmap and calls
grid.on_padding_click() via parent walk (signals + event filters
both failed on Wayland/QScrollArea)
- Grid tracks a pending rubber band origin; only activates past 30px
manhattan distance so small clicks deselect cleanly
- Move/release events forwarded from ThumbnailWidget to grid for both
the pending-drag check and the active rubber band drag
- Fixed mapFrom/mapTo direction (mapFrom's first arg must be a parent)