fix(engine): settings sync section scrollable + flaky midnight test

Add 96px bottom padding to SettingsPanelScrollable so the Sync section
is fully reachable by scrolling on Android (was clipped at container edge).

Fix check_system_fires_warning_event_only_once_per_day flakiness: Bevy
0.18 Messages<T> keeps events visible for two frames, so tests running
near UTC midnight saw a stale WarningToastEvent from headless_app()'s
initial update. Clear the buffer with .clear() before each assertion.

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

solitaire_engine/src/settings_plugin.rs

@@ -1481,6 +1481,7 @@ fn spawn_settings_panel(
                 row_gap: VAL_SPACE_3,
                 max_height: Val::Vh(60.0),
                 overflow: Overflow::scroll_y(),
+                padding: UiRect::bottom(Val::Px(96.0)),
                 ..default()
             },
         ))