Fix due command undercounting due cards

[oddish3]

print_due called Database::due_on, which matches only cards whose due_date is exactly the given date.

This misses never-reviewed cards (due_date is NULL) and overdue cards (due_date before the given date), both of which a drill session would still pull in via all_due.

Switching due to use all_due so it reports what a drill session would actually select.

[eudoxia0]

Ok so New cards are kind of an edge case.

If I'm doing hashcards due . today, yes, I want cards due today, before today, or new cards.

But if I'm doing hashcards due . tomorrow or some specific date, I don't want new cards, or overdue cards, I want only those who due date is that date, exactly.

[oddish3]

Fair point.
Updated due_report to use all_due (overdue + never-reviewed + due-today) only when date == Date::today(), and fall back to the exact due_on match for any other date. Added a regression test for the future-date case alongside the existing one.