Fix AttributeError on startup: resolve App.slot_var via winfo_toplevel

ScreenshotFrame previously read master.master.slot_var, but
master is the Notebook and master.master is the inner padding Frame
inside App._build, not App. Use winfo_toplevel() so we always reach
the App where slot_var lives.

Reported as: AttributeError: 'Frame' object has no attribute 'slot_var'
at gui.py:401 during run.py startup.

sephiria_inv/gui.py

@@ -398,7 +398,7 @@ class ScreenshotFrame(ttk.Frame):
     def __init__(self, master, on_confirmed) -> None:
         super().__init__(master, padding=6)
         self.on_confirmed = on_confirmed
-        self.slot_var = master.master.slot_var  # type: ignore[attr-defined]
+        self.slot_var = master.winfo_toplevel().slot_var  # type: ignore[attr-defined]
         self.image: Optional[Image.Image] = None
         self.bbox: Optional[Tuple[int, int, int, int]] = None
         self.cells: List[CellResult] = []