fix(multi): resolve 26 bugs found in comprehensive codebase review

Core fixes (issues #12, #13, #22):
- #12: undo now preserves score delta instead of restoring snapshot score
- #13: take_from_foundation defaults to false (non-standard house rule)
- #22: check_win validates full suit sequence, not just card count

Engine fixes:
- #8:  replay keyboard input guard against non-replay state
- #9:  help modal scrims.is_empty() guard added
- #10: settings modal scrims.is_empty() guard added
- #11: sync_plugin builds payload at poll time (not task-spawn time)
- #14: server replay mode case-sensitivity fix ("Classic")
- #15: play_by_seed_plugin confirmed flag set to true on launch
- #16: replay back-step debounce via Local<bool> + StateChangedEvent;
       register StateChangedEvent in ReplayOverlayPlugin (fixes 52 tests)
- #17: time-attack timer ignores win-summary overlay
- #18: HUD dropdown glyphs U+25BE → U+2193 (FiraMono-safe arrow)
- #19: theme plugin applies immediate visual update on A→B→A switch
- #20: SyncAuthError / SyncBusyOverlay split into separate entities so
       auth errors are visible after busy overlay is hidden
- #21: handle_forfeit ordered before update_stats_on_new_game
- #23: server merge uses correct avg_time_seconds and games_lost math
- #24: win_summary migrated to ModalScrim pattern
- #25: card_animation apply_deferred between animation systems
- #26: cursor_plugin HashMap access uses .get() with fallback
- #27: auto_complete mid-sequence deactivation guard
- #28: feedback_anim SettleAnim ordered before FoundationFlourish
- #29: achievement_plugin iterates all win events; adds scrims guard
- #30: leaderboard modal scrims.is_empty() guard added
- #31: server auth tmp file cleanup on rename failure
- #32: sync_setup modal scrims.is_empty() guard added
- #33: font_plugin uses match fallback; TokioRuntimeResource graceful
       current-thread fallback on runtime init failure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/leaderboard_plugin.rs

@@ -21,7 +21,7 @@ use crate::settings_plugin::{SettingsResource, SettingsStoragePath};
 use crate::sync_plugin::SyncProviderResource;
 use crate::ui_modal::{
     spawn_modal, spawn_modal_actions, spawn_modal_button, spawn_modal_header, ButtonVariant,
-    ScrimDismissible,
+    ModalScrim, ScrimDismissible,
 };
 use crate::ui_theme::{
     ACCENT_PRIMARY, BG_ELEVATED, BORDER_SUBTLE, RADIUS_SM, STATE_INFO,
@@ -714,6 +714,7 @@ fn data_cell(
 fn handle_set_display_name_button(
     button_q: Query<&Interaction, (Changed<Interaction>, With<SetDisplayNameButton>)>,
     existing: Query<(), With<DisplayNameModal>>,
+    other_modal_scrims: Query<(), (With<ModalScrim>, Without<DisplayNameModal>)>,
     mut commands: Commands,
     settings: Option<Res<SettingsResource>>,
     font_res: Option<Res<FontResource>>,
@@ -725,6 +726,9 @@ fn handle_set_display_name_button(
     if !existing.is_empty() {
         return; // already open
     }
+    if !other_modal_scrims.is_empty() {
+        return; // Another modal is already visible.
+    }
     buf.0 = settings
         .as_ref()
         .and_then(|s| s.0.leaderboard_display_name.clone())