默认主题改为浅色 + 管理员可配置全局默认主题

- system_settings 新增 AppearanceSettings(default_theme)，默认为 light，
  对应 GET /api/system-settings/appearance（任何人可读）+ PUT（admin 可写）
- App.tsx 把 NextThemesProvider 的 defaultTheme 从 dark 改为 light，并把
  QueryClientProvider 提到 ThemeProvider 外层以便后者使用 useQuery
- ThemeContext 新增 localStorage 标志 strategy-theme-user-set：用户调用
  toggleTheme / setTheme 时打标；每次启动若标志未设，则跟随 admin 配置的
  default_theme 同步主题，admin 改动用户刷新即生效；标志一旦存在永远尊重
  用户个人选择
- legacy 'theme' 键迁移视为用户显式选择，避免老用户被新默认覆盖
- frontend system-settings api/hook 增 appearance 拉取与更新

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend-web/src/contexts/ThemeContext.tsx

@@ -7,6 +7,8 @@ import React, {
 } from 'react';
 import { useTheme as useNextTheme } from 'next-themes';
 
+import { useAppearanceSettings } from '@/core/system-settings/hooks';
+
 // 主题类型
 export type ThemeType = 'dark' | 'light';
 
@@ -83,13 +85,29 @@ const themes: Record<ThemeType, ThemeConfig> = {
 };
 
 const STORAGE_KEY = 'strategy-theme';
+// Marker that the user has explicitly toggled the theme (via the appearance
+// settings page or any future toggle UI). When this flag is absent we treat
+// any value stored in ``strategy-theme`` as one we put there ourselves while
+// syncing the admin's default — so we'll re-sync on every boot, picking up
+// admin changes. When the flag is present we never override the user.
+const EXPLICIT_FLAG_KEY = 'strategy-theme-user-set';
 
 function readPersistedTheme(): ThemeType {
   if (typeof window === 'undefined') {
-    return 'dark';
+    return 'light';
   }
   const saved = localStorage.getItem(STORAGE_KEY);
-  return saved === 'light' ? 'light' : 'dark';
+  return saved === 'dark' ? 'dark' : 'light';
+}
+
+function hasExplicitChoice(): boolean {
+  if (typeof window === 'undefined') return false;
+  return localStorage.getItem(EXPLICIT_FLAG_KEY) === '1';
+}
+
+function markExplicitChoice(): void {
+  if (typeof window === 'undefined') return;
+  localStorage.setItem(EXPLICIT_FLAG_KEY, '1');
 }
 
 // 主题上下文
@@ -112,42 +130,68 @@ interface ThemeProviderProps {
 export const ThemeProvider: React.FC<ThemeProviderProps> = ({ children }) => {
   const { resolvedTheme, setTheme: setNextTheme } = useNextTheme();
   const [mounted, setMounted] = useState(false);
+  // Admin-configured global default. If the user hasn't explicitly chosen
+  // a theme yet, this value drives the rendered theme — and re-syncs on
+  // every mount so an admin change propagates without users clearing
+  // localStorage by hand.
+  const { data: appearance } = useAppearanceSettings();
 
   useEffect(() => {
     setMounted(true);
   }, []);
 
-  // 迁移 / 规范化 localStorage，避免与 next-themes 冲突导致闪回浅色
+  // 迁移 / 规范化 localStorage：去掉旧的 'theme' 键，并清掉一切非法值。
+  // 注意：不要在这里强制写入一个 fallback —— 那会让 next-themes 视作
+  // 「用户已选过」，admin 默认就再也注入不进来了。
   useEffect(() => {
     if (!mounted) return;
     const current = localStorage.getItem(STORAGE_KEY);
     const legacy = localStorage.getItem('theme');
 
-    if (current !== 'light' && current !== 'dark') {
-      const fallback =
-        legacy === 'light' || legacy === 'dark' ? legacy : 'dark';
-      setNextTheme(fallback);
-    } else if (!current && legacy && (legacy === 'light' || legacy === 'dark')) {
-      setNextTheme(legacy);
+    if (current && current !== 'light' && current !== 'dark') {
+      // Garbage value — wipe it so the boot-sync below can take over.
+      localStorage.removeItem(STORAGE_KEY);
+    }
+    if (!current && legacy && (legacy === 'light' || legacy === 'dark')) {
+      // Migrate the legacy key as the user's explicit choice (it was set
+      // long enough ago that we should preserve their pick rather than
+      // override with admin default).
+      setNextTheme(legacy);
+      markExplicitChoice();
     }
-
     if (legacy) {
       localStorage.removeItem('theme');
     }
   }, [mounted, setNextTheme]);
 
+  // Boot-time sync: when the user has never picked a theme themselves,
+  // apply whatever the admin configured. Re-runs whenever the admin
+  // setting changes (so a logged-in user sees the new default after
+  // refreshing without having to clear cookies).
+  useEffect(() => {
+    if (!mounted) return;
+    if (hasExplicitChoice()) return;
+    const adminDefault = appearance?.default_theme;
+    if (!adminDefault) return;
+    if (resolvedTheme !== adminDefault) {
+      setNextTheme(adminDefault);
+    }
+  }, [mounted, appearance?.default_theme, resolvedTheme, setNextTheme]);
+
   const theme: ThemeType = mounted
-    ? resolvedTheme === 'light'
-      ? 'light'
-      : 'dark'
+    ? resolvedTheme === 'dark'
+      ? 'dark'
+      : 'light'
     : readPersistedTheme();
 
   const toggleTheme = () => {
     setNextTheme(theme === 'dark' ? 'light' : 'dark');
+    markExplicitChoice();
   };
 
   const setTheme = (newTheme: ThemeType) => {
     setNextTheme(newTheme);
+    markExplicitChoice();
   };
 
   const themeConfig = themes[theme];