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/challenge_plugin.rs

@@ -54,11 +54,10 @@ fn advance_on_challenge_win(
         }
         let prev = progress.0.challenge_index;
         progress.0.challenge_index = prev.saturating_add(1);
-        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 after challenge advance: {e}");
             }
-        }
         // Human-readable level is 1-based (index 0 → "Challenge 1").
         let level_number = prev.saturating_add(1);
         toast.write(InfoToastEvent(format!("Challenge {level_number} complete!")));