style: cargo fmt under rustfmt 1.9 and gate formatting in CI

The repo was formatted under an older stable; rustfmt 1.9 (Rust 1.95)
wraps signatures and call sites differently, so every touched file was
picking up unrelated formatting hunks. One mechanical pass, and a
'cargo fmt --check' step in the test workflow (same pinned 1.95.0
toolchain) so drift can't accumulate again.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/theme/plugin.rs

@@ -252,8 +252,6 @@ fn apply_theme_to_card_image_set(theme: &CardTheme, image_set: &mut CardImageSet
     image_set.theme_back = Some(theme.back.clone());
 }
 
-
-
 /// Switches the active theme to the one served at
 /// `themes://<theme_id>/theme.ron`. Returns the new `Handle<CardTheme>`
 /// so callers can poll `Assets<CardTheme>` if they want to wait for