solitaire: detect a won board and finish it in one press

A drained, all-face-up board is a guaranteed clear that auto() sweeps home
in a single cascade, but the only way to trigger it was double-clicking
thirty cards home by hand. Add State.Won(), surface it in the view, and
swap the ordinary controls for one pulsing finish button when it's true.

internal/games/klondike/klondike.go

@@ -620,6 +620,31 @@ func (s State) Net() int64 {
 // cleared reports whether every card is home.
 func (s State) cleared() bool { return s.Home() == FullDeck }
 
+// Won reports that the board is a guaranteed clear: nothing left in the stock or
+// waste, and not a single face-down card under any column. From here every card
+// is a face-up run and a single auto() drains the whole board to the foundations
+// without a choice left to make — which is exactly the tedious tail the felt
+// should offer to finish in one gesture instead of thirty double-clicks.
+//
+// It's deliberately narrower than "solvable": a draw-one board with cards still
+// in the stock is winnable too, but auto() won't turn the stock over, so calling
+// that won would light a finish button that then stalls. This is the state the
+// finish button actually finishes.
+func (s State) Won() bool {
+	if s.Phase == PhaseDone || s.cleared() {
+		return false
+	}
+	if len(s.Stock) > 0 || len(s.Waste) > 0 {
+		return false
+	}
+	for _, p := range s.Table {
+		if len(p.Down) > 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // CanAuto reports whether anything can go home at all — which is what greys the
 // finish button out rather than letting it be pressed at a board that has nothing
 // for it.