feat(engine): convert PauseScreen to modal + add ForfeitConfirmScreen

Pause overlay drops its bespoke full-screen layout and is rebuilt on
the standard `ui_modal` scaffold: uniform scrim, centred card, real
Resume (Primary, Esc) and Forfeit (Tertiary, G) action buttons. The
Draw Mode row stays inline in the body so the existing toggle still
fires `SettingsChangedEvent`.

The G-key double-press toast countdown is replaced with a real
modal: `G` (or clicking Forfeit on Pause) fires the new
`ForfeitRequestEvent`, which `PausePlugin` answers by spawning
`ForfeitConfirmScreen` at `Z_PAUSE_DIALOG` (above pause). The modal
exposes Cancel + "Yes, forfeit" buttons plus Y/Enter/N/Esc
accelerators; confirmation despawns both modals, clears
`PausedResource`, and fires `ForfeitEvent` for `StatsPlugin`.

`toggle_pause` now early-returns when a forfeit modal is visible (and
runs `.before(handle_forfeit_keyboard)`) so an Esc that closes the
forfeit modal doesn't also re-open pause in the same frame.

The legacy `forfeit_countdown` field, `FORFEIT_CONFIRM_WINDOW`
constant, and the six pure-function countdown tests are removed; new
tests cover the modal-spawn / confirm / cancel paths and the active-
game predicate that still gates the G hotkey.

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

solitaire_engine/src/events.rs

@@ -189,6 +189,15 @@ pub struct XpAwardedEvent {
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct ForfeitEvent;
 
+/// Request to open the forfeit-confirm modal. Fired by the `G` accelerator
+/// and by the Pause modal's "Forfeit" button so the same modal opens
+/// either way. Consumed by `PausePlugin`, which spawns
+/// `ForfeitConfirmScreen` after checking that a game is in progress and
+/// no forfeit modal is already showing. Confirmation inside that modal
+/// then fires `ForfeitEvent` for `StatsPlugin` to consume.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct ForfeitRequestEvent;
+
 /// Fired when the player requests a hint (H key). Carries the source card ID
 /// and destination pile for visual highlighting.
 ///