fix(engine): settings panel scrollable via touch on Android

scroll_settings_panel only read MouseWheel, which is generated by desktop
scroll wheels and two-finger OS-level scroll gestures. On Android, a
single-finger swipe generates TouchInput, not MouseWheel, leaving the
settings panel unscrollable on real touchscreen devices.

Added touch_scroll_settings_panel: tracks touch start Y, applies the
vertical delta from each Moved event to ScrollPosition, resets on lift.
Registered TouchInput messages in SettingsPlugin::build so tests that use
MinimalPlugins (which omit InputPlugin) don't fail with "Message not
initialized".

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

solitaire_engine/src/settings_plugin.rs

@@ -12,6 +12,7 @@
 use std::path::PathBuf;
 
 use bevy::input::mouse::{MouseScrollUnit, MouseWheel};
+use bevy::input::touch::{TouchInput, TouchPhase};
 use bevy::prelude::*;
 use bevy::ui::{ComputedNode, UiGlobalTransform};
 use bevy::window::{WindowMoved, WindowResized};
@@ -357,6 +358,7 @@ impl Plugin for SettingsPlugin {
             .add_message::<ToggleSettingsRequestEvent>()
             .add_message::<InfoToastEvent>()
             .add_message::<bevy::input::mouse::MouseWheel>()
+            .add_message::<bevy::input::touch::TouchInput>()
             // `WindowResized` / `WindowMoved` are real Bevy window events
             // and emitted by the windowing backend under `DefaultPlugins`,
             // but we register them explicitly here so the geometry watcher
@@ -369,6 +371,7 @@ impl Plugin for SettingsPlugin {
                     handle_volume_keys,
                     toggle_settings_screen,
                     scroll_settings_panel,
+                    touch_scroll_settings_panel,
                     record_window_geometry_changes,
                     persist_window_geometry_after_debounce,
                 ),
@@ -1367,6 +1370,43 @@ fn scroll_settings_panel(
     }
 }
 
+/// Scrolls the settings panel in response to touch pan gestures (Android).
+/// Tracks the most recent touch Y so each `Moved` event's delta can be
+/// applied to `ScrollPosition`. `MouseWheel` handles desktop; this system
+/// fills the gap for single-finger swipe on touchscreen devices.
+fn touch_scroll_settings_panel(
+    mut touch_evr: MessageReader<TouchInput>,
+    screen: Res<SettingsScreen>,
+    mut scrollables: Query<&mut ScrollPosition, With<SettingsPanelScrollable>>,
+    mut last_y: Local<Option<f32>>,
+) {
+    if !screen.0 {
+        touch_evr.clear();
+        *last_y = None;
+        return;
+    }
+    for event in touch_evr.read() {
+        match event.phase {
+            TouchPhase::Started => {
+                *last_y = Some(event.position.y);
+            }
+            TouchPhase::Moved => {
+                if let Some(prev) = *last_y {
+                    let delta = event.position.y - prev;
+                    for mut sp in scrollables.iter_mut() {
+                        // Swiping up (delta < 0) scrolls content down (sp.y increases).
+                        sp.0.y = (sp.0.y - delta).max(0.0);
+                    }
+                }
+                *last_y = Some(event.position.y);
+            }
+            TouchPhase::Ended | TouchPhase::Canceled => {
+                *last_y = None;
+            }
+        }
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Window geometry persistence
 // ---------------------------------------------------------------------------