Fix saving to an icloud folder. - #317

I had set an icloud folder is the default save location, but after a
while the app stopped saving tracking files.  I think there was a
permissions error, but the error was swallowed.

I'm not an iOS developer at all, so I don't know if this is the correct
fix.  This is what Claude told me:

> When you first pick the folder in the document picker, iOS hands the
> app a URL with a live access grant that stays valid for the rest of that
> app run. So in that session — and until you fully quit the app —
> saveToURL can write to the iCloud folder even without unlocking it. It
> genuinely works.

> After you force-quit and relaunch, that live grant is gone. Now the
> only way back to the folder is the persisted bookmark
> (Preferences.swift:343). Resolving the bookmark gives you the URL back,
> but on iOS a resolved bookmark URL is locked until you call
> startAccessingSecurityScopedResource(). And notice the getter doesn't
> hold that open — it only unlocks briefly if the bookmark is stale
> (:351-359), then immediately re-locks via defer.

This might fix merlos#306, but I'm not sure how to test.