feat(ai): 统一 AI 配置入口到 /admin/ai-settings

## 新增
- 创建 /admin/ai-settings 统一配置页（AiProviderSettingsCard + AiUsageDashboard）
- admin 侧边栏新增"AI 配置"菜单项（权限 AI_CONFIGURE，图标 Sparkles）
- 新增 deleteAiProvider 数据访问层（事务删除 + 自动转移默认）
- 新增 deleteAiProviderAction Server Action（Zod 校验 + 权限校验）
- AiProviderSettingsCard 新增删除按钮（AlertDialog 确认 + destructive 变体）
- 新增 i18n 翻译键（delete/deleteConfirm/deleteSuccess 等，zh-CN + en）

## 移除
- 从 /settings 移除 AI 标签页（原 VALID_TABS 含 "ai"，现仅 4 标签页）
- 从考试页面移除 AI 配置弹窗（Dialog + AiProviderSettingsCard 内嵌）
- 从 ai-provider-selector.tsx 移除配置弹窗（managePanel/manageOpen props）
- 移除 settings-view.tsx 中 canConfigureAi 逻辑和未使用 import

## 变更
- 考试页面"管理"按钮改为 Link 跳转到 /admin/ai-settings
- ai-provider-selector.tsx"管理"按钮改为 Link 跳转到 /admin/ai-settings
- exam-form.tsx 移除 providerDialogOpen/providerDialogKey 状态
- 修正架构文档 004 中 Action 命名（getAiProvidersAction → getAiProviderSummaries 等）

## 架构文档同步
- 004 更新 settings 模块章节（V3 标记/修正 Action 名称/新增 deleteAiProvider）
- 005 新增 deleteAiProviderAction 节点 + /admin/ai-settings 路由

src/shared/i18n/messages/en/settings.json

@@ -227,7 +227,15 @@
       "saveFailure": "Failed to save",
       "loadFailure": "Failed to load AI providers",
       "needKey": "Please enter API key to test",
-      "needTest": "Please test the configuration before saving"
+      "needTest": "Please test the configuration before saving",
+      "delete": "Delete",
+      "deleteNeedSelect": "Please select a provider to delete",
+      "deleteSuccess": "Provider deleted",
+      "deleteFailure": "Failed to delete provider",
+      "deleteConfirmTitle": "Confirm deletion",
+      "deleteConfirmDescription": "This action cannot be undone. If the deleted provider was the default, the most recent provider will be set as default automatically.",
+      "deleteConfirm": "Delete",
+      "deleteCancel": "Cancel"
     }
   },
   "quickLinks": {

src/shared/i18n/messages/zh-CN/settings.json

@@ -227,7 +227,15 @@
       "saveFailure": "保存失败",
       "loadFailure": "加载 AI 服务商失败",
       "needKey": "请输入 API 密钥进行测试",
-      "needTest": "保存前请先测试配置"
+      "needTest": "保存前请先测试配置",
+      "delete": "删除",
+      "deleteNeedSelect": "请先选择要删除的服务商",
+      "deleteSuccess": "已删除服务商",
+      "deleteFailure": "删除失败",
+      "deleteConfirmTitle": "确认删除",
+      "deleteConfirmDescription": "此操作不可撤销。删除后若该服务商为默认，将自动选择最新的服务商作为默认。",
+      "deleteConfirm": "确认删除",
+      "deleteCancel": "取消"
     }
   },
   "quickLinks": {