refactor(core): derive Copy for DrawMode; drop redundant .clone() calls (M-18)

DrawMode is a fieldless two-variant enum — it is trivially bitwise-
copyable. Adding Copy + updating choose_winnable_seed to take the value
directly eliminates 13 superfluous .clone() calls across solitaire_core,
solitaire_engine, solitaire_assetgen, and solitaire_wasm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/play_by_seed_plugin.rs

@@ -338,7 +338,7 @@ fn tick_debounce_and_spawn_solver_task(
 
     let draw_mode = settings
         .as_ref()
-        .map_or(DrawMode::DrawOne, |s| s.0.draw_mode.clone());
+        .map_or(DrawMode::DrawOne, |s| s.0.draw_mode);
     let cfg = SolverConfig::default();
     let task = AsyncComputeTaskPool::get()
         .spawn(async move { try_solve(seed, draw_mode, &cfg) });