feat(web): 整体替换 UI 框架为 Ant Design 6 并接入自定义主题

以 antd ConfigProvider + antd-style 承载 zinc 风格自定义主题（明暗双套 token），
button/input/select/notification 用 createStyles 定制；三态主题经
darkAlgorithm 无刷新切换；use-toast 改基于 antd notification；
移除 shadcn/ui 组件目录与 radix 依赖；样式桥接层保证排版与多端布局不变。

web/src/pages/Login.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react'
-import { Link, useNavigate } from 'react-router'
-import { ArrowLeft, ArrowRight, CircleAlert, Eye, EyeOff, Lock } from 'lucide-react'
+import { useNavigate } from 'react-router'
+import { ArrowLeft, ArrowRight, Eye, EyeOff, Lock } from 'lucide-react'
+import { Alert, Button, Card, Input } from 'antd'
 import { useAuth } from '../lib/auth'
 import { ApiError } from '../lib/api'
 import { useDocumentTitle } from '../features/site'
@@ -45,28 +46,28 @@ export default function Login() {
         <span className="pn-login-wordmark">Pure Note</span>
       </div>
 
-      <form className="card pn-login-card" onSubmit={onSubmit}>
-        <h1 className="pn-login-title">管理员登录</h1>
-        <p className="pn-login-sub">请输入管理密码以继续</p>
+      <Card className="pn-login-card" styles={{ body: { padding: 0 } }}>
+        <form onSubmit={onSubmit}>
+          <h1 className="pn-login-title">管理员登录</h1>
+          <p className="pn-login-sub">请输入管理密码以继续</p>
 
-        <label htmlFor="pn-login-username" className="sr-only">用户名</label>
-        <input
-          id="pn-login-username"
-          className="sr-only"
-          type="text"
-          name="username"
-          autoComplete="username"
-          value="admin"
-          readOnly
-          tabIndex={-1}
-        />
-
-        <label htmlFor="pn-login-password" className="sr-only">管理密码</label>
-        <div className="pn-login-field">
-          <Lock size={16} className="pn-login-icon" aria-hidden />
+          <label htmlFor="pn-login-username" className="sr-only">用户名</label>
           <input
+            id="pn-login-username"
+            className="sr-only"
+            type="text"
+            name="username"
+            autoComplete="username"
+            value="admin"
+            readOnly
+            tabIndex={-1}
+          />
+
+          <label htmlFor="pn-login-password" className="sr-only">管理密码</label>
+          <Input
             id="pn-login-password"
-            className="pn-login-input"
+            size="large"
+            variant="outlined"
             type={showPw ? 'text' : 'password'}
             name="password"
             placeholder="请输入管理密码"
@@ -75,36 +76,47 @@ export default function Login() {
             required
             value={password}
             onChange={(e) => setPassword(e.target.value)}
+            prefix={<Lock size={16} className="pn-login-lock" aria-hidden />}
+            suffix={
+              <Button
+                type="text"
+                size="small"
+                aria-label={showPw ? '隐藏密码' : '显示密码'}
+                onClick={() => setShowPw((v) => !v)}
+                icon={showPw ? <EyeOff size={16} /> : <Eye size={16} />}
+              />
+            }
           />
-          <button
-            type="button"
-            className="pn-login-eye"
-            aria-label={showPw ? '隐藏密码' : '显示密码'}
-            onClick={() => setShowPw((v) => !v)}
+
+          {error && (
+            <Alert role="alert" type="error" showIcon message={error} className="pn-login-alert" />
+          )}
+
+          <Button
+            htmlType="submit"
+            type="primary"
+            size="large"
+            block
+            className="mt-5"
+            disabled={submitting || password.length === 0}
           >
-            {showPw ? <EyeOff size={16} /> : <Eye size={16} />}
-          </button>
-        </div>
+            <span>{submitting ? '登录中…' : '登录'}</span>
+            <ArrowRight size={16} />
+          </Button>
 
-        {error && (
-          <div className="pn-login-error" role="alert">
-            <CircleAlert size={15} />
-            <span>{error}</span>
-          </div>
-        )}
+          <p className="pn-login-hint">连续失败将被暂时锁定</p>
+        </form>
+      </Card>
 
-        <button type="submit" className="btn primary pn-login-submit" disabled={submitting || password.length === 0}>
-          <span>{submitting ? '登录中…' : '登录'}</span>
-          <ArrowRight size={16} />
-        </button>
-
-        <p className="pn-login-hint">连续失败将被暂时锁定</p>
-      </form>
-
-      <Link to="/" className="btn ghost pn-login-back">
-        <ArrowLeft size={14} />
-        <span>返回博客</span>
-      </Link>
+      <Button
+        type="text"
+        size="small"
+        className="pn-login-back"
+        icon={<ArrowLeft size={14} />}
+        onClick={() => navigate('/')}
+      >
+        返回博客
+      </Button>
     </div>
   )
 }