We ask if the user wants to install Pandoc every time they try to import/export (if pypandoc is present).

Behavior needs to be smarter. Here's the goal:

If no PyPandoc installed:

1) Just use the limited fallback import/export silently

If PyPandoc is installed, but Pandoc is missing:

1) Prompt the first time (existing behavior), save result (new)

2) If no or fail, add a menu item to retry later but never prompt again.

3) If yes and success, all good!

Ask once, then save the answer as a pref so we know we already asked.