feat(02-01): brand auth pages with gradient bg, wordmark, and Alert errors

- LoginPage: pastel gradient background (saving/bill/investment light shades)
- LoginPage: gradient text wordmark with data-testid='wordmark'
- LoginPage: shadcn Alert destructive with AlertCircle icon for error display
- RegisterPage: mirrors LoginPage branding treatment exactly
- Both pages: Card shadow-lg for visual lift against gradient
- All 6 AUTH tests pass (AUTH-01 through AUTH-04)

frontend/src/pages/LoginPage.tsx

@@ -1,8 +1,11 @@
 import { useState } from 'react'
 import { useTranslation } from 'react-i18next'
+import { AlertCircle } from 'lucide-react'
 import { Button } from '@/components/ui/button'
-import { Card, CardContent, CardDescription, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
+import { Card, CardContent, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
 import { Input } from '@/components/ui/input'
+import { Alert, AlertDescription } from '@/components/ui/alert'
+import { palette } from '@/lib/palette'
 import type { AuthContext } from '@/hooks/useAuth'
 
 interface Props {
@@ -31,15 +34,36 @@ export function LoginPage({ auth: { login }, onToggle }: Props) {
   }
 
   return (
-    <div className="flex min-h-screen items-center justify-center bg-background">
-      <Card className="w-full max-w-md">
+    <div
+      className="flex min-h-screen items-center justify-center"
+      style={{
+        background: `linear-gradient(135deg, ${palette.saving.light}, ${palette.bill.light}, ${palette.investment.light})`,
+      }}
+    >
+      <Card className="w-full max-w-md shadow-lg">
         <CardHeader>
           <CardTitle>{t('auth.login')}</CardTitle>
-          <CardDescription>{t('app.title')}</CardDescription>
+          <span
+            data-testid="wordmark"
+            className="text-2xl font-bold tracking-tight"
+            style={{
+              background: `linear-gradient(to right, oklch(0.50 0.12 260), oklch(0.50 0.12 320))`,
+              WebkitBackgroundClip: 'text',
+              WebkitTextFillColor: 'transparent',
+              backgroundClip: 'text',
+            }}
+          >
+            {t('app.title')}
+          </span>
         </CardHeader>
         <form onSubmit={handleSubmit}>
           <CardContent className="flex flex-col gap-4">
-            {error && <p className="text-sm text-destructive">{error}</p>}
+            {error && (
+              <Alert variant="destructive">
+                <AlertCircle className="h-4 w-4" />
+                <AlertDescription>{error}</AlertDescription>
+              </Alert>
+            )}
             <Input
               type="email"
               placeholder={t('auth.email')}

frontend/src/pages/RegisterPage.tsx

@@ -1,8 +1,11 @@
 import { useState } from 'react'
 import { useTranslation } from 'react-i18next'
+import { AlertCircle } from 'lucide-react'
 import { Button } from '@/components/ui/button'
-import { Card, CardContent, CardDescription, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
+import { Card, CardContent, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
 import { Input } from '@/components/ui/input'
+import { Alert, AlertDescription } from '@/components/ui/alert'
+import { palette } from '@/lib/palette'
 import type { AuthContext } from '@/hooks/useAuth'
 
 interface Props {
@@ -32,15 +35,36 @@ export function RegisterPage({ auth: { register }, onToggle }: Props) {
   }
 
   return (
-    <div className="flex min-h-screen items-center justify-center bg-background">
-      <Card className="w-full max-w-md">
+    <div
+      className="flex min-h-screen items-center justify-center"
+      style={{
+        background: `linear-gradient(135deg, ${palette.saving.light}, ${palette.bill.light}, ${palette.investment.light})`,
+      }}
+    >
+      <Card className="w-full max-w-md shadow-lg">
         <CardHeader>
           <CardTitle>{t('auth.register')}</CardTitle>
-          <CardDescription>{t('app.title')}</CardDescription>
+          <span
+            data-testid="wordmark"
+            className="text-2xl font-bold tracking-tight"
+            style={{
+              background: `linear-gradient(to right, oklch(0.50 0.12 260), oklch(0.50 0.12 320))`,
+              WebkitBackgroundClip: 'text',
+              WebkitTextFillColor: 'transparent',
+              backgroundClip: 'text',
+            }}
+          >
+            {t('app.title')}
+          </span>
         </CardHeader>
         <form onSubmit={handleSubmit}>
           <CardContent className="flex flex-col gap-4">
-            {error && <p className="text-sm text-destructive">{error}</p>}
+            {error && (
+              <Alert variant="destructive">
+                <AlertCircle className="h-4 w-4" />
+                <AlertDescription>{error}</AlertDescription>
+              </Alert>
+            )}
             <Input
               placeholder={t('auth.displayName')}
               value={displayName}