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/leaderboard_plugin.rs

@@ -862,10 +862,8 @@ fn handle_display_name_confirm(
                 .leaderboard_display_name
                 .clone()
                 .unwrap_or_else(|| {
-                    if let SyncBackend::SolitaireServer {
-                        ref username,
-                        ..
-                    } = settings.0.sync_backend
+                    if let SyncBackend::SolitaireServer { ref username, .. } =
+                        settings.0.sync_backend
                     {
                         username.chars().take(32).collect()
                     } else {