fix(engine): resolve all clippy warnings introduced by PNG asset pipeline

- Collapse nested-if patterns into let-chains across 13 plugins (42 instances)
- Add #[allow(clippy::too_many_arguments)] to 5 Bevy systems in card_plugin
  and input_plugin where ECS parameter count exceeds the lint threshold
- Gate Theme import in table_plugin under #[cfg(test)] — only used by
  test-only colour helpers; removing the unconditional import silences the
  unused-import lint without breaking the test suite
- Wrap ButtonInput<MouseButton> in Option<> in update_input_platform so that
  tests using MinimalPlugins (no InputPlugin) no longer panic on startup

All 789 tests pass; cargo clippy --workspace -- -D warnings is clean.

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

solitaire_engine/src/card_plugin.rs

@@ -394,6 +394,7 @@ fn sync_cards_startup(
     }
 }
 
+#[allow(clippy::too_many_arguments)]
 fn sync_cards_on_change(
     mut events: MessageReader<StateChangedEvent>,
     commands: Commands,
@@ -416,6 +417,7 @@ fn sync_cards_on_change(
     }
 }
 
+#[allow(clippy::too_many_arguments)]
 fn sync_cards(
     mut commands: Commands,
     game: &GameState,
@@ -542,6 +544,7 @@ fn face_colour(card: &Card, color_blind: bool) -> Color {
     }
 }
 
+#[allow(clippy::too_many_arguments)]
 fn spawn_card_entity(
     commands: &mut Commands,
     card: &Card,