fix: duplicate a standard page as a plain page

[surajshetty3416]

Fixes #597

frappe.copy_doc keeps no_copy fields unless told otherwise, so duplicating a standard page carried is_standard and app over. Outside developer mode the copy then opened read-only, and inside it a second export was written for the same app. The duplicate now starts as a plain page: not standard, no app.

[greptile-apps]

Confidence Score: 5/5

Safe to merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.89%. Comparing base (7d1ac04) to head (184d625).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #811      +/-   ##
===========================================
+ Coverage    66.86%   66.89%   +0.02%     
===========================================
  Files           94       94              
  Lines        10496    10505       +9     
===========================================
+ Hits          7018     7027       +9     
  Misses        3478     3478              

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