feat(engine): unify destructive-confirm verbs — drop "Yes," prefix

Both confirm modals previously used a "Yes, <verb>" pattern that read
like a question-and-answer dialog ("Are you sure? Yes, forfeit"); the
canonical UX pattern for a destructive confirm is just the bare verb.

The Confirm New Game modal's primary button is now "New game" instead
of "Yes, abandon" — matching the verb the user originally clicked
and framing the action positively rather than as a loss.

The Forfeit Confirm modal's primary button is now "Forfeit" instead
of "Yes, forfeit" — same pattern, less ceremony.

The Pause menu's own Resume / Forfeit buttons are unchanged: it's an
action menu, not a destructive confirm, and bare verbs are already
correct there.

Two doc comments and the ui_modal.rs spawn_modal_button example
docstring are updated to reflect the new copy. Marker symbol names
(ConfirmYesButton, ForfeitConfirmButton) are kept to avoid
unnecessary churn — the rename would ripple into mouse-input handlers
without a matching user-visible benefit.

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

solitaire_engine/src/ui_modal.rs

@@ -38,7 +38,7 @@
 //!             spawn_modal_button(
 //!                 actions,
 //!                 ConfirmButton,
-//!                 "Yes, abandon",
+//!                 "New game",
 //!                 Some("Y"),
 //!                 ButtonVariant::Primary,
 //!                 font_res,