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/sync_plugin.rs

@@ -176,21 +176,18 @@ fn poll_pull_result(
             let (merged, _conflicts) = merge(&local, &remote);
 
             // Persist merged state atomically.
-            if let Some(p) = &stats_path.0 {
-                if let Err(e) = save_stats_to(p, &merged.stats) {
+            if let Some(p) = &stats_path.0
+                && let Err(e) = save_stats_to(p, &merged.stats) {
                     warn!("sync: failed to persist stats: {e}");
                 }
-            }
-            if let Some(p) = &achievements_path.0 {
-                if let Err(e) = save_achievements_to(p, &merged.achievements) {
+            if let Some(p) = &achievements_path.0
+                && let Err(e) = save_achievements_to(p, &merged.achievements) {
                     warn!("sync: failed to persist achievements: {e}");
                 }
-            }
-            if let Some(p) = &progress_path.0 {
-                if let Err(e) = save_progress_to(p, &merged.progress) {
+            if let Some(p) = &progress_path.0
+                && let Err(e) = save_progress_to(p, &merged.progress) {
                     warn!("sync: failed to persist progress: {e}");
                 }
-            }
 
             // Update in-world resources.
             stats.0 = merged.stats;