增加自动亮色/暗色主题切换 + 清理 pyc 文件

- 新建 store/theme.tsx: 亮色/暗色/跟随系统三种模式
- App.tsx: ThemeProvider + ConfigProvider 暗色算法
- MainLayout: 侧边栏主题切换按钮，Sider/Content 随主题变化
- 所有页面硬编码颜色替换为 antd token
- AssignmentsPage: Tag 用预设颜色名适配暗色，未选中背景用 colorBgContainer
- LoginPage: 暗色深色渐变背景
- 从 git 移除所有 __pycache__/*.pyc 文件

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/frontend/src/App.tsx

@@ -1,6 +1,6 @@
 import { useState, useCallback } from 'react';
 import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom';
-import { ConfigProvider } from 'antd';
+import { ConfigProvider, theme } from 'antd';
 import zhCN from 'antd/locale/zh_CN';
 import LoginPage from './pages/LoginPage';
 import MainLayout from './layouts/MainLayout';
@@ -12,15 +12,25 @@ import ProxyPage from './pages/ProxyPage';
 import UsersPage from './pages/UsersPage';
 import CookiePage from './pages/CookiePage';
 import { getToken } from './store/auth';
+import { ThemeProvider, useTheme } from './store/theme';
 
-function App() {
+function AppContent() {
   // 用 state 驱动重渲染，登录/登出时调 refreshAuth()
   const [authVersion, setAuthVersion] = useState(0);
   const refreshAuth = useCallback(() => setAuthVersion((v) => v + 1), []);
   const isLoggedIn = !!getToken();
+  const { isDark } = useTheme();
 
   return (
-    <ConfigProvider locale={zhCN}>
+    <ConfigProvider
+      locale={zhCN}
+      theme={{
+        algorithm: isDark ? theme.darkAlgorithm : theme.defaultAlgorithm,
+        token: {
+          borderRadius: 6,
+        },
+      }}
+    >
       <BrowserRouter>
         <Routes>
           <Route path="/login" element={<LoginPage onLogin={refreshAuth} />} />
@@ -43,4 +53,12 @@ function App() {
   );
 }
 
+function App() {
+  return (
+    <ThemeProvider>
+      <AppContent />
+    </ThemeProvider>
+  );
+}
+
 export default App;