style: declare HoverableButton state attribute for the pyright gate

The dynamic _beamline_state assignment was a pre-existing pattern, but
the panel rewrite turned its line into a changed line, so the
basedpyright diff gate now counts it. Declare the attribute instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/aare/gui/panels/beamline_state_panel.py

@@ -63,8 +63,11 @@ class HoverableButton(QPushButton):
     hovered = Signal(object)
     unhovered = Signal()
 
+    # Set by the panel right after construction; carried in hover signals.
+    _beamline_state: BeamlineStateEnum | None = None
+
     def enterEvent(self, event) -> None:
-        self.hovered.emit(getattr(self, "_beamline_state", None))
+        self.hovered.emit(self._beamline_state)
         super().enterEvent(event)
 
     def leaveEvent(self, event) -> None: