Unify popup UI to TradeAlertModal design system.

Add AppPopup shell, shared CSS, MUI theme overrides, and center positioning for consistent modal styling across frontend and frontend_golden.

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

frontend_golden/src/components/LoginDialog.tsx

@@ -1,7 +1,6 @@
 import React, { useState } from 'react';
 import {
   Dialog,
-  DialogTitle,
   DialogContent,
   DialogActions,
   TextField,
@@ -14,9 +13,10 @@ import {
   InputAdornment,
   Divider,
 } from '@mui/material';
-import { Visibility, VisibilityOff, Close, AccountCircle } from '@mui/icons-material';
+import { Visibility, VisibilityOff, AccountCircle } from '@mui/icons-material';
 import { useAuth } from '../contexts/AuthContext';
 import DraggablePaper from './DraggablePaper';
+import GoldenPopupDialogTitle from './GoldenPopupDialogTitle';
 
 interface LoginDialogProps {
   open: boolean;
@@ -87,32 +87,15 @@ const LoginDialog: React.FC<LoginDialogProps> = ({ open, onClose, onSwitchToSign
         pointerEvents: 'none',
         '& .MuiDialog-container': {
           pointerEvents: 'none',
-          alignItems: 'flex-start',
+          alignItems: 'center',
+          justifyContent: 'center',
         },
       }}
       PaperProps={{
-        sx: {
-          pointerEvents: 'auto',
-          boxShadow: '0 8px 32px rgba(0, 0, 0, 0.3)',
-          border: '2px solid rgba(80, 100, 160, 0.5)',
-        }
+        sx: { pointerEvents: 'auto' },
       }}
     >
-      <DialogTitle sx={{
-        display: 'flex',
-        alignItems: 'center',
-        justifyContent: 'space-between',
-        bgcolor: '#1976d2',
-        color: '#fff',
-        py: 1.2,
-      }}>
-        <Typography variant="h6" sx={{ fontWeight: 600, fontSize: '16px', color: '#fff' }}>
-          🔐 로그인
-        </Typography>
-        <IconButton size="small" onClick={handleClose} sx={{ color: '#fff' }}>
-          <Close />
-        </IconButton>
-      </DialogTitle>
+      <GoldenPopupDialogTitle badge="AUTH" title="LOGIN" titleKo="로그인" onClose={handleClose} />
 
       <form onSubmit={handleSubmit}>
         <DialogContent>