feat(全栈): 实现简易模式核心功能

**功能概述**：
实现简易模式(Simple Mode)，为个人用户和小团队提供简化的使用体验，隐藏复杂的分组、订阅、配额等概念。

**后端改动**：
1. 配置系统
   - 新增 run_mode 配置项（standard/simple）
   - 支持环境变量 RUN_MODE
   - 默认值为 standard

2. 数据库初始化
   - 自动创建3个默认分组：anthropic-default、openai-default、gemini-default
   - 默认分组配置：无并发限制、active状态、非独占
   - 幂等性保证：重复启动不会重复创建

3. 账号管理
   - 创建账号时自动绑定对应平台的默认分组
   - 如果未指定分组，自动查找并绑定默认分组

**前端改动**：
1. 状态管理
   - authStore 新增 isSimpleMode 计算属性
   - 从后端API获取并同步运行模式

2. UI隐藏
   - 侧边栏：隐藏分组管理、订阅管理、兑换码菜单
   - 账号管理页面：隐藏分组列
   - 创建/编辑账号对话框：隐藏分组选择器

3. 路由守卫
   - 限制访问分组、订阅、兑换码相关页面
   - 访问受限页面时自动重定向到仪表板

**配置示例**：
```yaml
run_mode: simple

run_mode: standard
```

**影响范围**：
- 后端：配置、数据库迁移、账号服务
- 前端：认证状态、路由、UI组件
- 部署：配置文件示例

**兼容性**：
- 简易模式和标准模式可无缝切换
- 不需要数据迁移
- 现有数据不受影响

frontend/src/components/account/CreateAccountModal.vue

@@ -964,8 +964,13 @@
         </div>
       </div>
 
-      <!-- Group Selection -->
-      <GroupSelector v-model="form.group_ids" :groups="groups" :platform="form.platform" />
+      <!-- Group Selection - 仅标准模式显示 -->
+      <GroupSelector
+        v-if="!authStore.isSimpleMode"
+        v-model="form.group_ids"
+        :groups="groups"
+        :platform="form.platform"
+      />
 
     </form>
 
@@ -1076,6 +1081,7 @@
 import { ref, reactive, computed, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { useAppStore } from '@/stores/app'
+import { useAuthStore } from '@/stores/auth'
 import { adminAPI } from '@/api/admin'
 import {
   useAccountOAuth,
@@ -1102,6 +1108,7 @@ interface OAuthFlowExposed {
 }
 
 const { t } = useI18n()
+const authStore = useAuthStore()
 
 const oauthStepTitle = computed(() => {
   if (form.platform === 'openai') return t('admin.accounts.oauth.openai.title')

frontend/src/components/account/EditAccountModal.vue

@@ -466,8 +466,13 @@
         <Select v-model="form.status" :options="statusOptions" />
       </div>
 
-      <!-- Group Selection -->
-      <GroupSelector v-model="form.group_ids" :groups="groups" :platform="account?.platform" />
+      <!-- Group Selection - 仅标准模式显示 -->
+      <GroupSelector
+        v-if="!authStore.isSimpleMode"
+        v-model="form.group_ids"
+        :groups="groups"
+        :platform="account?.platform"
+      />
 
     </form>
 
@@ -513,6 +518,7 @@
 import { ref, reactive, computed, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { useAppStore } from '@/stores/app'
+import { useAuthStore } from '@/stores/auth'
 import { adminAPI } from '@/api/admin'
 import type { Account, Proxy, Group } from '@/types'
 import BaseDialog from '@/components/common/BaseDialog.vue'
@@ -535,6 +541,7 @@ const emit = defineEmits<{
 
 const { t } = useI18n()
 const appStore = useAppStore()
+const authStore = useAuthStore()
 
 // Platform-specific hint for Base URL
 const baseUrlHint = computed(() => {

frontend/src/components/layout/AppSidebar.vue

@@ -432,8 +432,8 @@ const adminNavItems = computed(() => {
   const baseItems = [
     { path: '/admin/dashboard', label: t('nav.dashboard'), icon: DashboardIcon },
     { path: '/admin/users', label: t('nav.users'), icon: UsersIcon, hideInSimpleMode: true },
-    { path: '/admin/groups', label: t('nav.groups'), icon: FolderIcon },
-    { path: '/admin/subscriptions', label: t('nav.subscriptions'), icon: CreditCardIcon },
+    { path: '/admin/groups', label: t('nav.groups'), icon: FolderIcon, hideInSimpleMode: true },
+    { path: '/admin/subscriptions', label: t('nav.subscriptions'), icon: CreditCardIcon, hideInSimpleMode: true },
     { path: '/admin/accounts', label: t('nav.accounts'), icon: GlobeIcon },
     { path: '/admin/proxies', label: t('nav.proxies'), icon: ServerIcon },
     { path: '/admin/redeem', label: t('nav.redeemCodes'), icon: TicketIcon, hideInSimpleMode: true },