Use animated show/hide for playlist widgets and harden fade animations. Replace direct setVisible signal handlers with lambdas that call set_widget_visible_animated, and add set_widget_visible_animated to route to fade-in/out. Fade methods now stop opposing animations, reuse existing QGraphicsOpacityEffect when present, preserve current opacity as start values, and set effect opacity when creating a new effect. Also catch possible RuntimeError when stopping animations and keep animation references on widgets to avoid premature GC.
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