style(app-hub): 헤더 톤과 프로필 등급 배지 배치를 정리

맥락:
- /app 헤더가 시작 카드 톤과 달라 시각 흐름이 끊기고, 멤버십 등급이 프로필 트리거 내부에 있어 위계가 어색했다.

변경사항:
- AppTopBar를 Start Ritual 카드와 같은 라이트 톤(bg-white/78, text-brand-dark)으로 맞췄다.
- 멤버십 등급을 독립 뷰로 분리하기 위해 entities/user에 MembershipTierBadge 컴포넌트를 추가했다.
- ProfileMenu 트리거에서 등급 배지를 제거하고, 아바타+이름만 남겨 정보 위계를 단순화했다.
- docs/session_brief.md, docs/90_current_state.md에 최신 상태를 반영했다.

검증:
- npx tsc --noEmit

세션-상태: /app 헤더와 프로필 영역의 톤/위계 정렬을 완료했다.
세션-다음: RoomSheet 인원수 기반 UI를 큐레이션형 정보로 치환할지 확정이 필요하다.
세션-리스크: 밝은 사진 배경과 라이트 헤더 조합에서 상단 경계 인지가 약해질 수 있다.

src/entities/user/index.ts

@@ -1,2 +1,3 @@
 export * from './model/mockUser';
 export * from './model/types';
+export * from './ui/MembershipTierBadge';

src/entities/user/ui/MembershipTierBadge.tsx

@@ -0,0 +1,21 @@
+import type { MembershipTier } from '@/entities/user/model/types';
+
+interface MembershipTierBadgeProps {
+  tier: MembershipTier;
+}
+
+const TIER_STYLES: Record<MembershipTier, string> = {
+  pro: 'border-brand-primary/28 bg-brand-primary/12 text-brand-primary/82',
+  normal: 'border-brand-dark/18 bg-white/68 text-brand-dark/66',
+  team: 'border-emerald-600/28 bg-emerald-500/10 text-emerald-700',
+};
+
+export const MembershipTierBadge = ({ tier }: MembershipTierBadgeProps) => {
+  return (
+    <span
+      className={`inline-flex items-center rounded-full border px-2.5 py-1 text-[10px] font-semibold tracking-[0.1em] ${TIER_STYLES[tier]}`}
+    >
+      {tier.toUpperCase()}
+    </span>
+  );
+};

src/features/profile-menu/ui/ProfileMenu.tsx

@@ -7,21 +7,16 @@ interface ProfileMenuProps {
 }
 
 export const ProfileMenu = ({ user, onLogout }: ProfileMenuProps) => {
-  const tierLabel = user.membershipTier.toUpperCase();
-
   return (
     <Dropdown
       align="right"
       trigger={
-        <span className="inline-flex items-center gap-2 rounded-full border border-white/20 bg-white/10 px-2.5 py-1.5 text-sm text-white/90">
-          <span className="inline-flex h-7 w-7 items-center justify-center rounded-full bg-sky-300/30 text-xs font-semibold text-sky-100">
+        <span className="inline-flex items-center gap-2 rounded-full border border-brand-dark/14 bg-white/72 px-2.5 py-1.5 text-sm text-brand-dark/92 shadow-[0_12px_24px_rgba(15,23,42,0.08)] backdrop-blur-sm">
+          <span className="inline-flex h-7 w-7 items-center justify-center rounded-full bg-brand-primary/14 text-xs font-semibold text-brand-primary/82">
             {user.avatarLabel}
           </span>
-          <span className="hidden items-center gap-1.5 sm:inline-flex">
-            <span className="inline-flex items-center rounded-full border border-sky-100/45 bg-sky-100/18 px-2 py-0.5 text-[10px] font-semibold tracking-[0.08em] text-sky-50">
-              {tierLabel}
-            </span>
-            <span>{user.name}</span>
+          <span className="hidden items-center sm:inline-flex">
+            <span className="text-brand-dark/78">{user.name}</span>
           </span>
         </span>
       }

src/widgets/app-top-bar/ui/AppTopBar.tsx

@@ -1,4 +1,4 @@
-import type { ViewerProfile } from '@/entities/user';
+import { MembershipTierBadge, type ViewerProfile } from '@/entities/user';
 import { ProfileMenu } from '@/features/profile-menu';
 
 interface AppTopBarProps {
@@ -9,15 +9,18 @@ interface AppTopBarProps {
 
 export const AppTopBar = ({ user, oneLiner, onLogout }: AppTopBarProps) => {
   return (
-    <header className="sticky top-0 z-20 border-b border-white/18 px-4 py-3 sm:px-6">
+    <header className="sticky top-0 z-20 border-b border-brand-dark/12 bg-white/78 px-4 py-3 text-brand-dark backdrop-blur-md sm:px-6">
       <div className="mx-auto flex w-full max-w-7xl items-center justify-between gap-4">
         <div className="flex items-center gap-2">
-          <p className="text-sm font-semibold tracking-tight text-white">VibeRoom</p>
+          <p className="text-sm font-semibold tracking-tight text-brand-dark">VibeRoom</p>
         </div>
 
-        <p className="hidden text-center text-sm text-white/75 md:block">{oneLiner}</p>
+        <p className="hidden text-center text-sm text-brand-dark/64 md:block">{oneLiner}</p>
 
-        <ProfileMenu user={user} onLogout={onLogout} />
+        <div className="flex items-center gap-2">
+          <MembershipTierBadge tier={user.membershipTier} />
+          <ProfileMenu user={user} onLogout={onLogout} />
+        </div>
       </div>
     </header>
   );