Export XLS: add timestamp to default filename

The save dialog now suggests e.g. normal_kits_20260911_143200.xlsx
so successive exports never collide.

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

manage_kits.py

@@ -16,6 +16,7 @@ declared starts unticked, and ticking it adds it at its newest build.
 """
 
 import os
+from datetime import datetime
 import tkinter as tk
 from tkinter import filedialog, messagebox, ttk
 
@@ -301,7 +302,8 @@ class KitsPanel:
         default_name = ""
         if app_state.current_file_path:
             base = os.path.splitext(os.path.basename(app_state.current_file_path))[0]
-            default_name = f"{base}_kits.xlsx"
+            stamp = datetime.now().strftime("%Y%m%d_%H%M%S")
+            default_name = f"{base}_kits_{stamp}.xlsx"
 
         path = filedialog.asksaveasfilename(
             title="Export Kit Table",