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/leaderboard_plugin.rs

@@ -123,15 +123,14 @@ fn toggle_leaderboard_screen(
     spawn_leaderboard_screen(&mut commands, data.0.as_deref());
 
     // Start a background fetch if not already in flight.
-    if task_res.0.is_none() {
-        if let Some(p) = provider {
+    if task_res.0.is_none()
+        && let Some(p) = provider {
             let provider = p.0.clone();
             let task = AsyncComputeTaskPool::get().spawn(async move {
                 provider.fetch_leaderboard().await.map_err(|e| e.to_string())
             });
             task_res.0 = Some(task);
         }
-    }
 }
 
 /// Poll the background fetch task; store results when complete.