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/stats_plugin.rs

@@ -326,8 +326,8 @@ fn spawn_stats_screen(
             }
 
             // Time Attack section
-            if let Some(ta) = time_attack {
-                if ta.active {
+            if let Some(ta) = time_attack
+                && ta.active {
                     let mins = (ta.remaining_secs / 60.0).floor() as u64;
                     let secs = (ta.remaining_secs % 60.0).floor() as u64;
                     root.spawn((
@@ -336,7 +336,6 @@ fn spawn_stats_screen(
                         TextColor(Color::srgb(1.0, 0.6, 0.2)),
                     ));
                 }
-            }
 
             // Dismiss hint
             root.spawn((