feat(engine): N keypress now opens the real Confirm/Cancel modal

Previously a first N press during an active game showed a "Press N
again" toast and started a 3-second countdown — a UI-first violation
since the only continuation was another keystroke. The HUD New Game
button already routed through `ConfirmNewGameScreen` with real Cancel
/ New game buttons; this change makes keyboard N do the same.

- handle_keyboard_core fires NewGameRequestEvent::default() directly;
  handle_new_game's existing active-game check spawns the modal.
- Shift+N keeps the keyboard power-user bypass (confirmed: true).
- N is suppressed while the confirm modal or restore prompt is open
  so those modals' own input handlers can process N (cancel /
  start-new-game) without us re-firing the same frame they close.
- KeyboardConfirmState, NEW_GAME_CONFIRM_WINDOW, NewGameConfirmEvent,
  and the "Press N again" toast handler are removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/events.rs

@@ -207,13 +207,6 @@ pub struct ToggleLeaderboardRequestEvent;
 #[derive(Message, Debug, Clone)]
 pub struct SyncCompleteEvent(pub Result<SyncResponse, String>);
 
-/// Fired by `InputPlugin` when N is pressed while a game is in progress
-/// but confirmation has not yet been received. The animation plugin shows
-/// a "Press N again to confirm" toast. A second N press within the
-/// confirmation window sends `NewGameRequestEvent`.
-#[derive(Message, Debug, Clone, Copy, Default)]
-pub struct NewGameConfirmEvent;
-
 /// Generic informational toast message. Any system can fire this to display
 /// a short string to the player, e.g. "Locked — reach level 5".
 #[derive(Message, Debug, Clone)]