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>

app_state.py

@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.174"
+VERSION = "0.0.0.175"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True
@@ -41,6 +41,9 @@ status_label = None      # the window's bottom status bar
 # colour wherever the change was made.
 DIRTY_TINT = "#ffe6e6"
 CLEAN_TINT = "white"
+# The wiresheet canvas around the sheet itself, clean and with unsaved changes.
+OFF_SHEET_TINT = "#f3f3f3"
+OFF_SHEET_DIRTY_TINT = "#f0dcdc"
 
 # The file name on the tree's root row turns this colour while anything is unwritten,
 # which is the one place a file name appears on screen -- a Tk window title cannot be
@@ -239,8 +242,7 @@ def mark_dirty():
     global is_dirty
     if not is_dirty:
         is_dirty = True
-        if canvas is not None:
-            canvas.config(bg=DIRTY_TINT)
+        paint_sheet_tint()
     refresh_dirty_display()
 
 
@@ -248,11 +250,26 @@ def mark_clean():
     """Clears the unsaved-state marker after a successful save or a fresh load."""
     global is_dirty
     is_dirty = False
-    if canvas is not None:
-        canvas.config(bg=CLEAN_TINT)
+    paint_sheet_tint()
     refresh_dirty_display()
 
 
+def sheet_tints():
+    """(sheet, off-sheet) colours for the wiresheet in the current saved/unsaved state."""
+    if is_dirty:
+        return DIRTY_TINT, OFF_SHEET_DIRTY_TINT
+    return CLEAN_TINT, OFF_SHEET_TINT
+
+
+def paint_sheet_tint():
+    """Recolours the sheet (the "sheet_bg" rectangle) and the canvas around it."""
+    if canvas is None:
+        return
+    sheet, off_sheet = sheet_tints()
+    canvas.config(bg=off_sheet)
+    canvas.itemconfig("sheet_bg", fill=sheet)
+
+
 # ── Live device connection state ────────────────────────────────
 live_mode = False
 device_session = None       # device_session.DeviceSession instance