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/ui_theme.rs

@@ -182,6 +182,10 @@ pub const Z_MODAL_SCRIM: i32 = 200;
 pub const Z_MODAL_PANEL: i32 = 210;
 /// Pause overlay outranks normal modals — pausing should always be on top.
 pub const Z_PAUSE: i32 = 220;
+/// Confirmation dialog stacked on top of the pause overlay (e.g. the
+/// forfeit-confirm modal launched from the pause modal). Sits above
+/// `Z_PAUSE` so the dialog is always visible over the paused state.
+pub const Z_PAUSE_DIALOG: i32 = 225;
 pub const Z_ONBOARDING: i32 = 230;
 /// Win cascade sits between modals and toasts so the celebration plays
 /// over a paused / mid-modal screen.
@@ -325,6 +329,7 @@ mod tests {
             Z_MODAL_SCRIM,
             Z_MODAL_PANEL,
             Z_PAUSE,
+            Z_PAUSE_DIALOG,
             Z_ONBOARDING,
             Z_WIN_CASCADE,
             Z_TOAST,