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_data/src/settings.rs

@@ -154,6 +154,14 @@ pub struct Settings {
     /// `#[serde(default)]`.
     #[serde(default)]
     pub reduce_motion_mode: bool,
+    /// When `true`, replaces translucent "glass" chrome (the floating
+    /// touch tab bar) with opaque fills so table content behind it can
+    /// never reduce label/icon contrast. The accessibility counterpart
+    /// of iOS "Reduce Transparency". Older `settings.json` files written
+    /// before this field existed deserialize cleanly to `false` thanks
+    /// to `#[serde(default)]`.
+    #[serde(default)]
+    pub reduce_transparency_mode: bool,
     /// Window size and screen position to restore on next launch. `None`
     /// means "use platform defaults" — set on first run, then populated
     /// as the player resizes / moves the window. Older `settings.json`
@@ -451,6 +459,7 @@ impl Default for Settings {
             color_blind_mode: false,
             high_contrast_mode: false,
             reduce_motion_mode: false,
+            reduce_transparency_mode: false,
             window_geometry: None,
             selected_theme_id: default_theme_id(),
             shown_achievement_onboarding: false,