feat: 实现公告板功能

- 后端：从 GitHub Issues API 获取公告数据
  - 支持筛选指定 assignees 的评论
  - 支持排除指定 issue 的评论
  - 从评论第一行提取标题（移除 Markdown 格式）
  - 支持获取 reactions 数据（+1、heart、laugh 等）
  - 实现 10 分钟缓存机制
  - 强制刷新时，如果被限流则保留缓存数据
  - 添加 GitHub API 健康检查

- 前端：公告列表和详情页面
  - 响应式设计（移动端和桌面端）
  - 移动端使用侧边抽屉显示列表
  - 支持 Markdown 渲染
  - 支持内容折叠（超过 30 行）
  - 显示 reactions（使用 emoji）
  - 添加刷新按钮（支持强制刷新）
  - 公告菜单项放在第一位
  - 默认页面改为公告页

- 其他：
  - 更新 favicon 为 SVG 格式
  - 添加 GitHub API 限流方案文档
  - 添加 GitHub Token 配置文档

frontend/src/locales/en/common.json

@@ -231,6 +231,7 @@
     "copyTradingConfig": "Copy Trading Config",
     "positions": "Position Management",
     "statistics": "Statistics",
+    "announcements": "Announcements",
     "users": "User Management",
     "systemSettings": "System",
     "systemOverview": "Overview",
@@ -952,5 +953,18 @@
     "webhookUrl": "Webhook URL",
     "webhookUrlPlaceholder": "Webhook URL (from Slack App settings)",
     "webhookUrlRequired": "Please enter Webhook URL"
+  },
+  "announcements": {
+    "title": "Announcements",
+    "noAnnouncements": "No announcements",
+    "noDetail": "Please select an announcement to view details",
+    "viewMore": "View more announcements",
+    "author": "Author",
+    "createdAt": "Published at",
+    "noTitle": "No title",
+    "expand": "Expand all",
+    "collapse": "Collapse",
+    "refresh": "Refresh",
+    "list": "List"
   }
 }

frontend/src/locales/zh-CN/common.json

@@ -148,6 +148,7 @@
     "copyTradingConfig": "跟单配置",
     "positions": "仓位管理",
     "statistics": "统计信息",
+    "announcements": "公告",
     "users": "用户管理",
     "systemSettings": "系统管理",
     "systemOverview": "概览",
@@ -869,5 +870,18 @@
     "webhookUrl": "Webhook URL",
     "webhookUrlPlaceholder": "Webhook URL（从 Slack App 设置中获取）",
     "webhookUrlRequired": "请输入 Webhook URL"
+  },
+  "announcements": {
+    "title": "公告",
+    "noAnnouncements": "暂无公告",
+    "noDetail": "请选择一条公告查看详情",
+    "viewMore": "查看更多公告",
+    "author": "作者",
+    "createdAt": "发布时间",
+    "noTitle": "无标题",
+    "expand": "展开全部",
+    "collapse": "收起",
+    "refresh": "刷新",
+    "list": "列表"
   }
 }

frontend/src/locales/zh-TW/common.json

@@ -231,6 +231,7 @@
     "copyTradingConfig": "跟單配置",
     "positions": "倉位管理",
     "statistics": "統計信息",
+    "announcements": "公告",
     "users": "用戶管理",
     "systemSettings": "系統管理",
     "systemOverview": "概覽",
@@ -952,5 +953,18 @@
     "webhookUrl": "Webhook URL",
     "webhookUrlPlaceholder": "Webhook URL（從 Slack App 設置中獲取）",
     "webhookUrlRequired": "請輸入 Webhook URL"
+  },
+  "announcements": {
+    "title": "公告",
+    "noAnnouncements": "暫無公告",
+    "noDetail": "請選擇一條公告查看詳情",
+    "viewMore": "查看更多公告",
+    "author": "作者",
+    "createdAt": "發布時間",
+    "noTitle": "無標題",
+    "expand": "展開全部",
+    "collapse": "收起",
+    "refresh": "刷新",
+    "list": "列表"
   }
 }