feat(engine): reduce-transparency + high-contrast support for glass chrome

New Settings::reduce_transparency_mode (serde-default, Accessibility
toggle row) swaps every GlassSurface's gradients for a flat opaque
BG_ELEVATED fill; high-contrast mode now also boosts the glass fill
opacity and rim luminance to BORDER_SUBTLE_HC levels. Both applied by
settings_plugin::update_glass_surfaces, which retargets the gradients
in place on toggle or on newly spawned glass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/settings_plugin/input.rs

@@ -353,6 +353,14 @@ pub(super) fn handle_settings_buttons(
                     **t = on_off_label(settings.0.reduce_motion_mode);
                 }
             }
+            SettingsButton::ToggleReduceTransparency => {
+                settings.0.reduce_transparency_mode = !settings.0.reduce_transparency_mode;
+                persist(&path, &settings.0);
+                changed.write(SettingsChangedEvent(settings.0.clone()));
+                // Text refreshed by `update_reduce_transparency_text`; the
+                // glass chrome itself follows via `update_glass_surfaces`
+                // next frame.
+            }
             SettingsButton::ToggleTouchInputMode => {
                 use solitaire_data::settings::TouchInputMode;
                 settings.0.touch_input_mode = match settings.0.touch_input_mode {