Wiresheet: grey margin strip, saved-state tint on and off the sheet

- The top/left margin no box may enter is grey and ungridded, so the white
  sheet starts where boxes can actually be placed.
- Saving/unsaved state no longer resets the canvas to white: the sheet goes
  white/pink and the area around it grey/greyish pink.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main.py

@@ -3743,7 +3743,7 @@ canvas_hscroll.pack(side=tk.BOTTOM, fill=tk.X)
 
 canvas = tk.Canvas(
     frame_editor,
-    bg=wiresheet.OFF_SHEET_COLOUR,
+    bg=app_state.OFF_SHEET_TINT,
     highlightthickness=0,
     xscrollcommand=canvas_hscroll.set,
     yscrollcommand=canvas_vscroll.set,