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/progress_plugin.rs

@@ -101,11 +101,10 @@ fn award_xp_on_win(
                 total_xp: progress.0.total_xp,
             });
         }
-        if let Some(target) = &path.0 {
-            if let Err(e) = save_progress_to(target, &progress.0) {
+        if let Some(target) = &path.0
+            && let Err(e) = save_progress_to(target, &progress.0) {
                 warn!("failed to save progress: {e}");
             }
-        }
     }
 }