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/table_plugin.rs

@@ -8,13 +8,14 @@ use bevy::prelude::*;
 use bevy::window::WindowResized;
 use solitaire_core::card::Suit;
 use solitaire_core::pile::PileType;
-use solitaire_data::settings::Theme;
 
 use crate::events::HintVisualEvent;
 use crate::layout::{compute_layout, Layout, LayoutResource};
 #[cfg(test)]
 use crate::layout::TABLE_COLOUR;
 use crate::settings_plugin::{SettingsChangedEvent, SettingsResource};
+#[cfg(test)]
+use solitaire_data::Theme;
 
 /// Holds pre-loaded [`Handle<Image>`]s for the 5 selectable table backgrounds.
 ///