tasks: count overdue by calendar day like the ranker does (V-581)

Stalls compared the due instant to now while Rank compares whole calendar days, so a task due at 09:00 was counted overdue from 09:01 while its own row on the same page still read the reason as today. Stalls now reads dayDelta.

A row with no capture time also counted as sitting, because the zero time is January of year 1 and every span from it clears ten days. Rank already guarded that and Stalls did not.

Folded the open-versus-candidate partition FormatRU and Spoken each carried into one split helper. The two have to agree on where that line falls.

internal/tasks/stall.go

@@ -56,10 +56,16 @@ func Stalls(items []Item, now time.Time) []Stall {
 			// meant to act on.
 			continue
 		}
-		if it.Due != nil && it.Due.Before(now) {
+		// Whole calendar days, the reading Rank already uses. An instant
+		// comparison calls a task due at 18:00 overdue from 18:01, so the count
+		// above the table would say "просрочено" beside a row whose own reason
+		// still said "сегодня".
+		if it.Due != nil && dayDelta(*it.Due, now) < 0 {
 			overdue++
 		}
-		if now.Sub(it.Created) >= StallDays*24*time.Hour {
+		// A row with no capture time has not sat for anything. Without the
+		// guard its zero time is January of year 1 and it always counts.
+		if !it.Created.IsZero() && now.Sub(it.Created) >= StallDays*24*time.Hour {
 			sitting++
 		}
 	}