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

Register touch_scroll_panel::<HelpScrollable> so the Controls overlay
can be scrolled by swipe on Android. Without it, the Mode Launcher and
Overlays sections (rows 2–19) were unreachable via touch.

Also add 96px bottom padding to HelpScrollable — same fix applied to
settings_plugin — so the last row clears the scroll-container edge.
Register TouchInput message so existing headless tests continue to pass.

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

solitaire_engine/src/help_plugin.rs

@@ -44,13 +44,19 @@ pub struct HelpPlugin;
 impl Plugin for HelpPlugin {
     fn build(&self, app: &mut App) {
         app.add_message::<HelpRequestEvent>()
-            // `MouseWheel` is emitted by Bevy's input plugin under
+            // `MouseWheel` and `TouchInput` are emitted by Bevy's input
-            // `DefaultPlugins`; register it explicitly so the help-scroll
+            // plugin under `DefaultPlugins`; register them explicitly so
-            // system also runs cleanly under `MinimalPlugins` in tests.
+            // scroll systems run cleanly under `MinimalPlugins` in tests.
             .add_message::<MouseWheel>()
+            .add_message::<bevy::input::touch::TouchInput>()
             .add_systems(
                 Update,
-                (toggle_help_screen, handle_help_close_button, scroll_help_panel),
+                (
+                    toggle_help_screen,
+                    handle_help_close_button,
+                    scroll_help_panel,
+                    crate::ui_modal::touch_scroll_panel::<HelpScrollable>,
+                ),
             );
     }
 }
@@ -229,6 +235,7 @@ fn spawn_help_screen(commands: &mut Commands, font_res: Option<&FontResource>) {
                 row_gap: VAL_SPACE_2,
                 max_height: Val::Vh(70.0),
                 overflow: Overflow::scroll_y(),
+                padding: UiRect::bottom(Val::Px(96.0)),
                 ..default()
             },
         ))