feat(engine): keyboard focus rings on modal buttons (Phase 1)

Every button spawned via spawn_modal_button is now keyboard-navigable.
Tab/Shift-Tab cycles focus within the active modal, Enter activates
the focused button via the same Interaction::Pressed signal mouse
clicks use, and the primary action auto-focuses on modal open. Mouse
clicks transfer focus so the two input modes stay in sync.

The visual indicator is a single overlay entity that's reparented
above the topmost modal scrim and tracks the focused button's
GlobalTransform + ComputedNode each frame. Sitting outside the
modal-card subtree means the ring isn't affected by the open
animation's 0.96→1.0 scale, and sitting outside any scroll container
means it can't be clipped by Settings' Overflow::scroll_y. Z-order
sits one rung above Z_MODAL_TOP via the new Z_FOCUS_RING token.

Existing 11 modals (Help, Stats, Achievements, Settings, Profile,
Leaderboard, Pause, Forfeit confirm, GameOver, Confirm new game,
Onboarding, Home) get focus support without any call-site changes —
attach_focusable_to_modal_buttons walks the ancestry of any
ModalButton lacking Focusable to find its scrim and tags it
automatically. selection_plugin's Tab handler keeps working when no
modal is open; when one is, focus consumes Tab/Enter before the
selection system sees them.

Phase 1 scope only — HUD action bar, Home mode cards, and Settings
bespoke buttons (icon, swatch, toggle) come in Phase 2/3.

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

solitaire_app/src/main.rs

@@ -10,8 +10,8 @@ use solitaire_engine::{
     CardPlugin, ChallengePlugin, CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin,
     FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
     OnboardingPlugin, PausePlugin, ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin,
-    StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, UiModalPlugin, WeeklyGoalsPlugin,
-    WinSummaryPlugin,
+    StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, UiFocusPlugin, UiModalPlugin,
+    WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
 fn main() {
@@ -99,6 +99,7 @@ fn main() {
         .add_plugins(LeaderboardPlugin)
         .add_plugins(WinSummaryPlugin)
         .add_plugins(UiModalPlugin)
+        .add_plugins(UiFocusPlugin)
         .run();
 }