用户中心 CSS 整包重命名：按 shell/credits/settings/responsive 职责拆分

废除 part-1..5 机械切片，重组成可维护的领域文件并修复历史拼接断点，index 同步改写。

assets/css/routes/user-center.css

@@ -1,5 +1,4 @@
-@import url("user-center/part-1.css");
-@import url("user-center/part-2.css");
-@import url("user-center/part-3.css");
-@import url("user-center/part-4.css");
-@import url("user-center/part-5.css");
+@import url("user-center/shell.css");
+@import url("user-center/credits.css");
+@import url("user-center/settings.css");
+@import url("user-center/responsive.css");