fix: 移除 GameMode 的 camelCase 重命名，修复前端传 Local/VsAi 无法匹配

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

core/src/types.rs

@@ -99,7 +99,6 @@ pub struct GameResult {
 
 /// 游戏模式 (Tauri IPC 兼容 — 纯标签, 不含字段)
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
-#[serde(rename_all = "camelCase")]
 pub enum GameMode {
     Local,
     VsAi,