feat(engine): require N-key confirmation when abandoning an active game

Pressing N during an active game (move_count > 0, not won) now shows a
"Press N again to start a new game" toast and only starts a new game if
N is pressed a second time within 3 seconds. Starting a fresh game or
pressing N after a win still acts immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/help_plugin.rs

@@ -44,7 +44,7 @@ fn spawn_help_screen(commands: &mut Commands) {
         "  Click stock  Draw".to_string(),
         String::new(),
         "-- New Game --".to_string(),
-        "  N            New Classic game".to_string(),
+        "  N            New Classic game (N twice if in progress)".to_string(),
         "  C            Start today's daily challenge".to_string(),
         "  Z            Start a Zen game (level 5+)".to_string(),
         "  X            Start the next Challenge (level 5+)".to_string(),