feat: 添加 Admin Dashboard — React 19 SPA，包含 7 个页面

- Dashboard: 健康状态轮询、状态卡片、内存统计、快捷操作
- Login: 二维码展示 + 3 秒自动轮询 + 倒计时 + 登出
- Browser: 探索/搜索/用户三标签页，Feed 网格、详情面板、评论树
- Publish: 图文/视频发布表单，支持标签、可见性、定时发布
- Interactions: 点赞/取消点赞、收藏、评论、回复 + 操作日志
- API Tester: 端点选择器、请求体编辑器、cURL 生成、响应查看、历史记录
- Settings: Token 配置、服务器 URL 设置

后端改动:
- app.ts: 生产环境提供 dist/web/ 静态文件服务 + SPA fallback
- Dockerfile: 添加 web 构建阶段
- package.json: 添加 build:web、build:all、dev:web 脚本

技术栈: React 19 + TypeScript + Vite 6 + Tailwind CSS（暗色主题）
产物: 85.5 KB gzip JS + 4 KB gzip CSS，零重型依赖

web/src/context/AuthContext.tsx

@@ -0,0 +1,37 @@
+import { createContext, useContext, useState, useCallback, type ReactNode } from 'react';
+
+interface AuthState {
+  token: string;
+  serverUrl: string;
+  setToken: (t: string) => void;
+  setServerUrl: (u: string) => void;
+}
+
+const AuthContext = createContext<AuthState | null>(null);
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [token, setTokenState] = useState(() => localStorage.getItem('smcp_token') || '');
+  const [serverUrl, setServerUrlState] = useState(() => localStorage.getItem('smcp_server_url') || '');
+
+  const setToken = useCallback((t: string) => {
+    localStorage.setItem('smcp_token', t);
+    setTokenState(t);
+  }, []);
+
+  const setServerUrl = useCallback((u: string) => {
+    localStorage.setItem('smcp_server_url', u);
+    setServerUrlState(u);
+  }, []);
+
+  return (
+    <AuthContext.Provider value={{ token, serverUrl, setToken, setServerUrl }}>
+      {children}
+    </AuthContext.Provider>
+  );
+}
+
+export function useAuth(): AuthState {
+  const ctx = useContext(AuthContext);
+  if (!ctx) throw new Error('useAuth must be used within AuthProvider');
+  return ctx;
+}

web/src/context/ToastContext.tsx

@@ -0,0 +1,67 @@
+import { createContext, useContext, useState, useCallback, type ReactNode } from 'react';
+
+type ToastType = 'success' | 'error' | 'info' | 'warning';
+
+interface Toast {
+  id: number;
+  type: ToastType;
+  message: string;
+}
+
+interface ToastState {
+  toasts: Toast[];
+  toast: (type: ToastType, message: string) => void;
+  dismiss: (id: number) => void;
+}
+
+const ToastContext = createContext<ToastState | null>(null);
+
+let nextId = 0;
+
+export function ToastProvider({ children }: { children: ReactNode }) {
+  const [toasts, setToasts] = useState<Toast[]>([]);
+
+  const dismiss = useCallback((id: number) => {
+    setToasts((prev) => prev.filter((t) => t.id !== id));
+  }, []);
+
+  const toast = useCallback(
+    (type: ToastType, message: string) => {
+      const id = nextId++;
+      setToasts((prev) => [...prev, { id, type, message }]);
+      setTimeout(() => dismiss(id), 4000);
+    },
+    [dismiss],
+  );
+
+  return (
+    <ToastContext.Provider value={{ toasts, toast, dismiss }}>
+      {children}
+      <div className="fixed bottom-4 right-4 z-50 flex flex-col gap-2 max-w-sm">
+        {toasts.map((t) => (
+          <div
+            key={t.id}
+            onClick={() => dismiss(t.id)}
+            className={`px-4 py-3 rounded-lg shadow-lg cursor-pointer text-sm font-medium transition-all ${
+              t.type === 'success'
+                ? 'bg-dark-success/20 text-dark-success border border-dark-success/30'
+                : t.type === 'error'
+                  ? 'bg-dark-danger/20 text-dark-danger border border-dark-danger/30'
+                  : t.type === 'warning'
+                    ? 'bg-dark-warning/20 text-dark-warning border border-dark-warning/30'
+                    : 'bg-dark-accent/20 text-dark-accent border border-dark-accent/30'
+            }`}
+          >
+            {t.message}
+          </div>
+        ))}
+      </div>
+    </ToastContext.Provider>
+  );
+}
+
+export function useToast(): ToastState {
+  const ctx = useContext(ToastContext);
+  if (!ctx) throw new Error('useToast must be used within ToastProvider');
+  return ctx;
+}