fix: 移除logo白色背景 + 优化登录页面UI和中文化

Logo修复:
  - 移除 logo.svg 中的白色背景矩形 (fill="#ffffff")
  - Logo 现在为透明背景，在深色主题上正确显示

登录页面优化:
  - 添加 "iAgent" 品牌标题 (28px, bold, 带字间距)
  - 副标题改为中文 "服务器集群运维智能体"
  - 表单中文化: 邮箱/密码/登录
  - 错误提示改为带背景色的卡片样式 (红色图标+文字)
  - 添加邮箱输入框 placeholder (user@example.com)
  - 密码框支持回车提交
  - 底部提示: "账号由管理员在后台创建或通过邀请链接注册"
  - 限制表单最大宽度 360px，外层改用 SingleChildScrollView 防溢出

用户账号说明:
  App 端不提供自助注册功能，用户账号通过以下方式创建:
  1. 管理员在 Web 后台 (用户管理页) 直接创建
  2. 管理员发送邀请链接，用户通过链接注册
  3. 通过 Web 端自助注册 (可选填公司名创建新租户)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

it0_app/assets/icons/logo.svg

@@ -1,7 +1,4 @@
 <svg width="400" height="400" viewBox="0 0 400 400" fill="none" xmlns="http://www.w3.org/2000/svg">
-  <!-- Background -->
-  <rect width="400" height="400" fill="#ffffff"/>
-
   <!-- Antenna stick -->
   <line x1="200" y1="88" x2="200" y2="58" stroke="#3CC98C" stroke-width="7" stroke-linecap="round"/>
   <!-- Antenna ball -->

it0_app/lib/features/auth/presentation/pages/login_page.dart

@@ -24,37 +24,53 @@ class _LoginPageState extends ConsumerState<LoginPage> {
     return Scaffold(
       backgroundColor: AppColors.background,
       body: Center(
-        child: Padding(
+        child: SingleChildScrollView(
-          padding: const EdgeInsets.all(32),
+          padding: const EdgeInsets.symmetric(horizontal: 32, vertical: 48),
           child: Form(
             key: _formKey,
+            child: ConstrainedBox(
+              constraints: const BoxConstraints(maxWidth: 360),
               child: Column(
                 mainAxisAlignment: MainAxisAlignment.center,
                 children: [
                   SvgPicture.asset(
                     'assets/icons/logo.svg',
-                  width: 100,
+                    width: 96,
-                  height: 100,
+                    height: 96,
                   ),
-                const SizedBox(height: 8),
+                  const SizedBox(height: 12),
                   const Text(
-                  'Operations Intelligent Agent',
+                    'iAgent',
-                  style: TextStyle(color: AppColors.textSecondary),
+                    style: TextStyle(
+                      color: AppColors.textPrimary,
+                      fontSize: 28,
+                      fontWeight: FontWeight.bold,
+                      letterSpacing: 2,
+                    ),
+                  ),
+                  const SizedBox(height: 4),
+                  const Text(
+                    '服务器集群运维智能体',
+                    style: TextStyle(
+                      color: AppColors.textSecondary,
+                      fontSize: 14,
+                    ),
                   ),
                   const SizedBox(height: 48),
                   TextFormField(
                     controller: _emailController,
                     keyboardType: TextInputType.emailAddress,
                     decoration: const InputDecoration(
-                    labelText: 'Email',
+                      labelText: '邮箱',
-                    prefixIcon: Icon(Icons.email),
+                      hintText: 'user@example.com',
+                      prefixIcon: Icon(Icons.email_outlined),
                     ),
                     validator: (value) {
                       if (value == null || value.isEmpty) {
-                      return 'Please enter your email';
+                        return '请输入邮箱地址';
                       }
                       if (!value.contains('@')) {
-                      return 'Please enter a valid email';
+                        return '请输入有效的邮箱地址';
                       }
                       return null;
                     },
@@ -64,26 +80,50 @@ class _LoginPageState extends ConsumerState<LoginPage> {
                     controller: _passwordController,
                     obscureText: true,
                     decoration: const InputDecoration(
-                    labelText: 'Password',
+                      labelText: '密码',
-                    prefixIcon: Icon(Icons.lock),
+                      prefixIcon: Icon(Icons.lock_outline),
                     ),
                     validator: (value) {
                       if (value == null || value.isEmpty) {
-                      return 'Please enter your password';
+                        return '请输入密码';
                       }
                       return null;
                     },
+                    onFieldSubmitted: (_) => _handleLogin(),
                   ),
                   if (authState.error != null) ...[
                     const SizedBox(height: 16),
-                  Text(
+                    Container(
+                      padding: const EdgeInsets.symmetric(
+                        horizontal: 12,
+                        vertical: 8,
+                      ),
+                      decoration: BoxDecoration(
+                        color: AppColors.error.withAlpha(25),
+                        borderRadius: BorderRadius.circular(8),
+                      ),
+                      child: Row(
+                        children: [
+                          const Icon(Icons.error_outline,
+                              color: AppColors.error, size: 18),
+                          const SizedBox(width: 8),
+                          Expanded(
+                            child: Text(
                               authState.error!,
-                    style: const TextStyle(color: Colors.red),
+                              style: const TextStyle(
+                                color: AppColors.error,
+                                fontSize: 13,
+                              ),
+                            ),
+                          ),
+                        ],
+                      ),
                     ),
                   ],
                   const SizedBox(height: 24),
                   SizedBox(
                     width: double.infinity,
+                    height: 48,
                     child: FilledButton(
                       onPressed: authState.isLoading ? null : _handleLogin,
                       child: authState.isLoading
@@ -95,14 +135,27 @@ class _LoginPageState extends ConsumerState<LoginPage> {
                                 color: Colors.white,
                               ),
                             )
-                        : const Text('Login'),
+                          : const Text(
+                              '登录',
+                              style: TextStyle(fontSize: 16),
                             ),
                     ),
+                  ),
+                  const SizedBox(height: 32),
+                  Text(
+                    '账号由管理员在后台创建或通过邀请链接注册',
+                    style: TextStyle(
+                      color: AppColors.textMuted,
+                      fontSize: 12,
+                    ),
+                    textAlign: TextAlign.center,
+                  ),
                 ],
               ),
             ),
           ),
         ),
+      ),
     );
   }