feat(themes): 新增查询用户已购主题接口

在 KeyboardThemePurchaseService 及其实现中增加 getUserPurchasedThemes 方法，
通过用户ID获取已支付主题列表；同步新增 /themes/purchased 接口并放行鉴权。
补充 KeyboardThemesRespVO 缺失的 themePreviewImageUrl 字段。

src/main/java/com/yolo/keyborad/controller/ThemesController.java

@@ -67,4 +67,12 @@ public class ThemesController {
         return ResultUtils.success(result);
     }
 
+    @GetMapping("/purchased")
+    @Operation(summary = "查询已购买的主题", description = "查询当前用户已购买的主题列表")
+    public BaseResponse<List<KeyboardThemesRespVO>> getPurchasedThemes() {
+        Long userId = StpUtil.getLoginIdAsLong();
+        List<KeyboardThemesRespVO> result = themePurchaseService.getUserPurchasedThemes(userId);
+        return ResultUtils.success(result);
+    }
+
 }