polish(frontend): 修正中文字重发虚，并疏朗首页与数据区

补齐 Noto 600/800 并关闭伪粗；Header 加大加粗导航并加板块入口；去掉灰块底、加大列表行距，站点数据格居中缩小数字。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/admin/AdminNav.tsx

@@ -159,7 +159,7 @@ export function AdminSidebar({ user }: { user: User }) {
               key={item.href}
               href={item.href}
               aria-current={active ? "page" : undefined}
-              className="flex items-center gap-2.5 px-3 py-2.5 rounded-xl text-[13.5px] transition-colors"
+              className="flex items-center gap-2.5 px-3 py-2.5 rounded-xl text-[13.5px] font-medium transition-colors"
               style={
                 active
                   ? { background: "var(--nav-active-bg)", color: "var(--nav-active-ink)" }
@@ -190,7 +190,7 @@ export function AdminSidebar({ user }: { user: User }) {
       <div className="flex items-center gap-2.5 mt-2 px-2.5 py-2.5 rounded-xl" style={{ background: "var(--panel-2)" }}>
         <Avatar name={user.nickname || user.username} src={user.avatar || undefined} size={32} />
         <div className="min-w-0 leading-tight">
-          <p className="text-[13px] truncate" style={{ color: "var(--ink)" }}>
+          <p className="text-[13px] font-medium truncate" style={{ color: "var(--ink)" }}>
             {user.nickname}
           </p>
           <p className="text-[11px] truncate" style={{ color: "var(--ink-3)" }}>