Fix missing checkbox image for unchecked required kit

The boxes dict was missing the (False, False) key — an unchecked kit
that is also required — which caused a KeyError when the Kit Manager
opened automatically for files with incomplete checksums.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

manage_kits.py

@@ -238,6 +238,7 @@ class KitsPanel:
             (True, True): make_checkbox_image(True, True),
             (False, True): make_checkbox_image(False, True),
             (True, False): make_checkbox_image(True, False),
+            (False, False): make_checkbox_image(False, False),
         }
 
         hint = ttk.Label(