feat(engine): UI scale setting — 90/100/115/130% (Phase K)

Settings -> Accessibility gains a UI Scale row cycling four steps,
applied live through bevy::ui::UiScale: every menu, modal, and HUD
element scales while the table itself stays window-fit via
compute_layout. New Settings::ui_scale (serde default 1.0, sanitized
clamp to [0.9, 1.3]).

Safe-area anchors and modal scrim padding pre-divide physical insets
by the UI scale so post-multiplication lands exactly on the system
bars — without this, 90% would sink the bottom action bar into the
Android gesture zone. The anchor systems also re-run on UiScale
changes, not just inset changes.

Deferred from Phase K (noted in the doc): seeding the setting from the
Android system font scale on first run (needs JNI), and the 44px
touch-target audit (on-device).

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

solitaire_engine/src/settings_plugin/updates.rs

@@ -515,6 +515,58 @@ pub(super) fn touch_input_mode_label(mode: &solitaire_data::settings::TouchInput
     }
 }
 
+/// The four UI-scale steps the settings row cycles through (Phase K).
+pub(super) const UI_SCALE_STEPS: [f32; 4] = [0.9, 1.0, 1.15, 1.3];
+
+/// The next UI-scale step after `current`, wrapping 130 % → 90 %. A
+/// hand-edited value between steps advances to the first step larger
+/// than it, so the cycle always makes visible progress.
+pub(super) fn next_ui_scale(current: f32) -> f32 {
+    for step in UI_SCALE_STEPS {
+        if step > current + 0.001 {
+            return step;
+        }
+    }
+    UI_SCALE_STEPS[0]
+}
+
+/// Display string for the UI-scale row, e.g. `"115%"`.
+pub(super) fn ui_scale_label(scale: f32) -> String {
+    format!("{:.0}%", scale * 100.0)
+}
+
+/// Refreshes the live UI-scale value text whenever settings change.
+pub(super) fn update_ui_scale_text(
+    settings: Res<SettingsResource>,
+    mut text_nodes: Query<&mut Text, With<UiScaleText>>,
+) {
+    if !settings.is_changed() {
+        return;
+    }
+    for mut text in &mut text_nodes {
+        **text = ui_scale_label(settings.0.ui_scale);
+    }
+}
+
+/// Applies `Settings::ui_scale` to the live [`bevy::ui::UiScale`]
+/// resource — at startup (first change tick) and whenever the setting
+/// changes. Absent under `MinimalPlugins` (no `bevy_ui`), hence the
+/// `Option`; the table is unaffected either way (`compute_layout`
+/// owns world-space sizing, not UI scale).
+pub(super) fn sync_ui_scale_resource(
+    settings: Res<SettingsResource>,
+    ui_scale: Option<ResMut<UiScale>>,
+) {
+    let Some(mut ui_scale) = ui_scale else { return };
+    if !settings.is_changed() {
+        return;
+    }
+    let target = settings.0.ui_scale;
+    if (ui_scale.0 - target).abs() > f32::EPSILON {
+        ui_scale.0 = target;
+    }
+}
+
 /// Display string for the "Smart window size" toggle. The argument
 /// is the *enabled* state (i.e. the inverse of the underlying
 /// `disable_smart_default_size` field) so reading the label gives