feat: 新增大量社区功能与基础组件，优化站点整体体验

- 新增事件总线式toast通知系统
- 新增头像、阅读进度、404/500错误页等基础组件
- 重构页脚、布局、分页、点赞按钮等现有组件
- 新增用户个人页、通知中心、帖子编辑/发布/置顶/推荐管理功能
- 优化登录注册页面样式与交互
- 修复路径类型文件引用，更新站点元信息与字体配置

frontend/app/layout.tsx

@@ -3,12 +3,16 @@ import { cookies } from "next/headers";
 import "./globals.css";
 import Header from "@/components/Header";
 import Footer from "@/components/Footer";
+import Toaster from "@/components/Toaster";
 
 export const metadata: Metadata = {
-  title: "姜十三论坛",
+  title: {
+    default: "姜十三论坛 · J13 FORUM",
+    template: "%s · 姜十三论坛",
+  },
   description: "姜十三论坛 - 技术分享与讨论社区",
   openGraph: {
-    title: "姜十三论坛",
+    title: "姜十三论坛 · J13 FORUM",
     description: "姜十三论坛 - 技术分享与讨论社区",
     type: "website",
   },
@@ -20,16 +24,33 @@ export default async function RootLayout({
   children: React.ReactNode;
 }) {
   const cookieStore = await cookies();
-  const theme = cookieStore.get("j13-theme")?.value || "light";
+  // 浅色优先的设计系统，仅在显式选择暗色时挂 .dark
+  const theme = cookieStore.get("j13-theme")?.value === "dark" ? "dark" : "light";
 
   return (
     <html lang="zh-CN" className={theme === "dark" ? "dark" : ""}>
+      <head>
+        <link rel="preconnect" href="https://fonts.googleapis.com" />
+        <link rel="preconnect" href="https://fonts.gstatic.com" crossOrigin="anonymous" />
+        <link
+          href="https://fonts.googleapis.com/css2?family=Plus+Jakarta+Sans:wght@400;500;600;700;800&family=JetBrains+Mono:wght@400;500;700&family=Noto+Sans+SC:wght@400;500;700;900&display=swap"
+          rel="stylesheet"
+        />
+      </head>
       <body className="min-h-screen flex flex-col">
+        <a href="#main" className="skip-link">
+          跳到主要内容
+        </a>
         <Header />
-        <main className="flex-1 max-w-5xl mx-auto w-full px-4 py-6">
+        <main
+          id="main"
+          tabIndex={-1}
+          className="flex-1 w-full max-w-6xl mx-auto px-4 sm:px-6 py-7 sm:py-9 outline-none"
+        >
           {children}
         </main>
         <Footer />
+        <Toaster />
       </body>
     </html>
   );