fix(frontend): keep user center and user home avatar frames rounded

The two large hero avatar frames go back to their legacy rounded-square look: the user center profile header keeps 6px and the public user home hero keeps var(--radius). Both declare an explicit radius because the global avatar primitive (img.avatar, 0,1,1) ties them on specificity and only the later route rule wins the cascade.

Everything else stays: the primitive still circles the bare avatar-mini in the ban and report modals, the VIP modal avatar and the post-card meta avatar, the friends page avatar is circular, and the VIP icon still leads the badge row. The contract test now guards the two hero frames as rounded-square so the primitive cannot silently re-circularize them.

assets/css/routes/user-home.css

@@ -88,7 +88,9 @@
 .aksrt-author-hero__avatar img {
   width: 120px;
   height: 120px;
-  border-radius: var(--radius-avatar, 50%);
+  /* 公开主页 hero 头像保持 legacy 圆角方形：显式声明圆角，避免被 public 捆绑里
+     同特异性（0,1,1）的全局头像原语 img.avatar 抢先。 */
+  border-radius: var(--radius);
   border: 4px solid var(--surface-1);
   box-shadow: 0 4px 12px rgba(0, 0, 0, 0.12);
   object-fit: cover;