perf: AI 搜索移到独立后台线程避免阻塞 GUI

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/src/ai/search.rs

@@ -5,6 +5,7 @@ use crate::rules;
 use crate::types::{Color, Position};
 
 /// Alpha-Beta AI 引擎
+#[derive(Clone)]
 pub struct AlphaBetaAi {
     depth: usize,
 }