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/animation_plugin.rs

@@ -472,13 +472,12 @@ fn drive_toast_display(
     }
 
     // If no active toast and the queue has messages, show the next one.
-    if active.entity.is_none() {
-        if let Some(message) = queue.0.pop_front() {
+    if active.entity.is_none()
+        && let Some(message) = queue.0.pop_front() {
             let entity = spawn_queued_toast(&mut commands, message);
             active.entity = Some(entity);
             active.timer = QUEUED_TOAST_SECS;
         }
-    }
 }
 
 /// Spawns a centered top-of-screen `ToastEntity` for the queued toast system.