style: 로그인 페이지 재설계 + 'GEN 3 ALGO' 태그라인 제거

- Login: 카드형 레이아웃, 키 아이콘·표시/숨김 토글, 인라인 에러(aria), 로딩 스피너,
  입력 시 에러 자동 해제, 그라디언트 텍스트 제거
- Layout 헤더: 태그라인 삭제, 브랜드명 단일 표기

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -8,7 +8,7 @@
  *
  * 구조:
  *   [Header] — sticky 상단 바 (z-50)
- *     - 브랜드 로고 (AutoStock GEN 3 ALGO)
+ *     - 브랜드 로고 (AutoStock)
  *     - 네비게이션 링크 (대시보드, 성과분석, 전략설정, 백테스트, 매매로그, MCP상태)
  *     - 연결 상태 표시 (Backend LIVE/OFFLINE, MCP connected/unknown)
  *   [Main] — 페이지 컨텐츠 영역
@@ -78,12 +78,7 @@ const Layout = ({ children }: LayoutProps) => {
                         <div className="bg-blue-600 p-2 rounded-lg shadow-lg shadow-blue-500/20">
                             <Activity className="text-white" size={20} />
                         </div>
-                        <div>
-                            <h1 className="text-xl font-bold bg-gradient-to-r from-blue-400 to-purple-400 bg-clip-text text-transparent leading-none">
-                                AutoStock
-                            </h1>
-                            <p className="text-[10px] text-gray-500 font-mono tracking-wider">GEN 3 ALGO</p>
-                        </div>
+                        <h1 className="text-lg font-semibold tracking-tight leading-none">AutoStock</h1>
                     </div>
 
                     {/* Navigation */}

frontend/src/pages/Login.tsx

@@ -10,7 +10,7 @@
  * - onLogin: 인증 성공 시 API 키를 전달받는 콜백 (App에서 localStorage 저장)
  */
 import { useState, FormEvent } from 'react'
-import { Activity } from 'lucide-react'
+import { Activity, Eye, EyeOff, KeyRound, Loader2 } from 'lucide-react'
 
 /** Login 컴포넌트 Props 정의 */
 interface LoginProps {
@@ -30,6 +30,7 @@ const Login = ({ onLogin }: LoginProps) => {
     const [apiKey, setApiKey] = useState('')
     const [error, setError] = useState('')
     const [loading, setLoading] = useState(false)
+    const [showKey, setShowKey] = useState(false)
 
     /**
      * 폼 제출 핸들러 — API 키 유효성 검증
@@ -77,48 +78,80 @@ const Login = ({ onLogin }: LoginProps) => {
     }
 
     return (
-        <div className="min-h-screen bg-[#0f1115] flex items-center justify-center">
-            <div className="w-full max-w-sm p-8">
-                <div className="flex flex-col items-center gap-4 mb-8">
-                    <div className="bg-blue-600 p-3 rounded-xl shadow-lg shadow-blue-500/20">
-                        <Activity className="text-white" size={28} />
-                    </div>
-                    <div className="text-center">
-                        <h1 className="text-2xl font-bold bg-gradient-to-r from-blue-400 to-purple-400 bg-clip-text text-transparent">
-                            AutoStock
-                        </h1>
-                        <p className="text-xs text-gray-500 font-mono mt-1">GEN 3 ALGO</p>
+        <div className="min-h-screen bg-[#0f1115] text-gray-100 flex items-center justify-center px-4 py-12">
+            <div className="w-full max-w-[380px]">
+                {/* 브랜드 */}
+                <div className="flex flex-col items-center mb-8">
+                    <div className="bg-blue-600 p-3 rounded-2xl shadow-lg shadow-blue-500/25 mb-4">
+                        <Activity className="text-white" size={26} />
                     </div>
+                    <h1 className="text-2xl font-semibold tracking-tight">AutoStock</h1>
+                    <p className="text-sm text-gray-500 mt-1">자동매매 대시보드</p>
                 </div>
 
-                <form onSubmit={handleSubmit} className="space-y-4">
+                {/* 로그인 카드 */}
+                <div className="bg-[#161b22] border border-gray-800 rounded-2xl p-6 shadow-xl shadow-black/30">
+                    <form onSubmit={handleSubmit} className="space-y-5" noValidate>
                         <div>
-                        <label className="block text-sm text-gray-400 mb-2">API Key</label>
+                            <label htmlFor="api-key" className="block text-sm font-medium text-gray-300 mb-2">
+                                API 키
+                            </label>
+                            <div className="relative">
+                                <KeyRound
+                                    size={16}
+                                    className="absolute left-3.5 top-1/2 -translate-y-1/2 text-gray-500 pointer-events-none"
+                                />
                                 <input
-                            type="password"
+                                    id="api-key"
+                                    type={showKey ? 'text' : 'password'}
                                     value={apiKey}
-                            onChange={(e) => setApiKey(e.target.value)}
-                            placeholder="SECRET_KEY 입력"
-                            className="w-full px-4 py-3 bg-gray-800/50 border border-gray-700 rounded-lg text-gray-100 placeholder-gray-600 focus:outline-none focus:border-blue-500 focus:ring-1 focus:ring-blue-500 transition-colors"
+                                    onChange={(e) => {
+                                        setApiKey(e.target.value)
+                                        if (error) setError('')
+                                    }}
+                                    placeholder="SECRET_KEY"
+                                    autoComplete="current-password"
+                                    spellCheck={false}
+                                    aria-invalid={!!error}
+                                    aria-describedby={error ? 'api-key-error' : undefined}
+                                    className={`w-full pl-10 pr-11 py-3 bg-[#0f1115] border rounded-xl text-gray-100 placeholder-gray-600 font-mono text-sm transition-colors focus:outline-none focus:ring-2 ${
+                                        error
+                                            ? 'border-red-500/60 focus:border-red-500 focus:ring-red-500/30'
+                                            : 'border-gray-700 focus:border-blue-500 focus:ring-blue-500/30'
+                                    }`}
                                     autoFocus
                                 />
+                                <button
+                                    type="button"
+                                    onClick={() => setShowKey((v) => !v)}
+                                    aria-label={showKey ? 'API 키 숨기기' : 'API 키 표시'}
+                                    tabIndex={-1}
+                                    className="absolute right-3 top-1/2 -translate-y-1/2 p-1 text-gray-500 hover:text-gray-300 transition-colors"
+                                >
+                                    {showKey ? <EyeOff size={16} /> : <Eye size={16} />}
+                                </button>
                             </div>
-
                             {error && (
-                        <p className="text-red-400 text-sm">{error}</p>
+                                <p id="api-key-error" role="alert" className="text-red-400 text-sm mt-2">
+                                    {error}
+                                </p>
                             )}
+                        </div>
 
                         <button
                             type="submit"
                             disabled={loading}
-                        className="w-full py-3 bg-blue-600 hover:bg-blue-500 disabled:bg-gray-700 disabled:text-gray-500 text-white font-medium rounded-lg transition-colors"
+                            className="w-full py-3 bg-blue-600 hover:bg-blue-500 active:bg-blue-700 disabled:bg-gray-700 disabled:text-gray-500 disabled:cursor-not-allowed text-white font-medium rounded-xl transition-colors flex items-center justify-center gap-2"
                         >
-                        {loading ? '확인 중...' : '로그인'}
+                            {loading && <Loader2 size={16} className="animate-spin" />}
+                            {loading ? '확인 중' : '로그인'}
                         </button>
                     </form>
+                </div>
 
-                <p className="text-xs text-gray-600 text-center mt-6">
-                    .env 파일의 SECRET_KEY 값을 입력하세요
+                <p className="text-xs text-gray-600 text-center mt-6 leading-relaxed">
+                    백엔드 <code className="font-mono text-gray-500">.env</code>의{' '}
+                    <code className="font-mono text-gray-500">SECRET_KEY</code> 값으로 로그인합니다.
                 </p>
             </div>
         </div>