feat: 实现自建情景功能

## 功能概述
- 用户可创建、编辑、删除自定义情景
- 支持自定义情景名称、图标、描述、Prompt、首句引导
- 完整的权限隔离，用户只能管理自己的情景
- 深度集成 Eino 框架，动态加载自建情景 Prompt

## 后端实现
### 数据库
- 新增 user_scenarios 表
- 支持用户配额（最多 20 个）
- 字段验证：description 可选，prompt 最小 10 字符

### API
- GET /api/scenarios - 获取用户情景列表
- POST /api/scenarios - 创建情景
- GET /api/scenarios/:id - 获取详情
- PATCH /api/scenarios/:id - 更新情景
- DELETE /api/scenarios/:id - 删除情景

### Eino 集成
- PipelineState 添加 UserID 字段
- nodes_history 动态加载用户自建情景
- GetScenarioPrompt 支持自建情景优先级

## 前端实现
### 组件
- CreateScenarioModal - 创建情景对话框
- EditScenarioModal - 编辑情景对话框
- ConfigPanel 改造 - 分组显示系统预置和自建情景

### Hook
- useScenarios - 合并系统和自建情景，提供 CRUD 接口

### 国际化
- 中文、英文、日文翻译支持

## 问题修复
- 修复 CORS 问题：使用 Vite 代理
- 统一验证规则：description 可选，prompt 最小 10 字符
- 修复数据库约束：使用 NULLIF 处理空字符串

## 文件变更
新增文件: 13 个
修改文件: 14 个

详见文档: docs/自建情景功能完整文档.md

frontend/src/lib/i18n/en-US.ts

@@ -178,4 +178,41 @@ export const enUS: TranslationMap = {
   // Account (in settings)
   "settings.account": "Account",
   "settings.account.user": "Current user",
+
+  // Custom scenarios
+  "settings.scenario.system": "System Scenarios",
+  "settings.scenario.custom": "My Scenarios",
+  "settings.scenario.empty": "No custom scenarios yet. Click the button above to create one.",
+  "scenario.create.button": "New Scenario",
+  "scenario.create.title": "Create Custom Scenario",
+  "scenario.edit.title": "Edit Scenario",
+  "scenario.create.name": "Scenario Name",
+  "scenario.create.namePlaceholder": "e.g. Creative Writing Coach",
+  "scenario.create.icon": "Icon",
+  "scenario.create.description": "Brief Description",
+  "scenario.create.descriptionPlaceholder": "One-line summary of this scenario",
+  "scenario.create.prompt": "System Prompt",
+  "scenario.create.promptPlaceholder": "Define the AI's role and interaction rules...",
+  "scenario.create.promptGuide": "View Guide",
+  "scenario.create.promptGuide.tips": "Writing tips:",
+  "scenario.create.promptGuide.tip1": "Clearly define the role: who you are, who you are not",
+  "scenario.create.promptGuide.tip2": "List interaction rules: how to respond, how many sentences",
+  "scenario.create.promptGuide.tip3": "Add constraints: what not to do",
+  "scenario.create.promptGuide.example": "Example:",
+  "scenario.create.greeting": "Greeting (Optional)",
+  "scenario.create.greetingPlaceholder": "The first message when switching to this scenario...",
+  "scenario.create.language": "Default Language",
+  "scenario.error.nameLength": "Scenario name must be 2-50 characters",
+  "scenario.error.promptLength": "Prompt must be 10-2000 characters",
+  "scenario.error.greetingLength": "Greeting must not exceed 500 characters",
+
+  // Common actions
+  "common.cancel": "Cancel",
+  "common.create": "Create",
+  "common.creating": "Creating...",
+  "common.save": "Save",
+  "common.saving": "Saving...",
+  "common.edit": "Edit",
+  "common.delete": "Delete",
+  "common.confirmDelete": "Click again to confirm",
 };