refactor: remove redundant Box wrappers

- dock/main.py: canvas_box eliminated, Revealer wraps canvas directly
- desktop/system.py: two single-child Box wrappers removed
- controlcenter/player.py: redundant album_cover wrapper removed
- controlcenter/expanded_player.py: same pattern as player.py

src/window/dock/main.py

@@ -28,14 +28,10 @@ class Dock(Window):
         self.set_app_paintable(True)
 
         self.canvas = DockCanvas(self)
-
-        canvas_box = Box(
-            name="dock-canvas-box",
-            children=[self.canvas],
-        )
+        self.canvas.set_name("dock-canvas-box")
 
         self.revealer = Revealer(
-            child=canvas_box,
+            child=self.canvas,
             transition_duration=200,
             transition_type="slide-up",
         )