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/App.tsx

@@ -26,18 +26,20 @@ function AppProviders({ children }: { children: React.ReactNode }) {
     <NextThemesProvider
       attribute="class"
       storageKey="strategy-theme"
-      defaultTheme="dark"
+      defaultTheme="light"
       enableSystem={false}
       disableTransitionOnChange
       forcedTheme={forcedTheme}
     >
-      <I18nProvider initialLocale={locale}>
-        <ThemeProvider>
-          <ChatProvider>
-            <QueryClientProvider>{children}</QueryClientProvider>
-          </ChatProvider>
-        </ThemeProvider>
-      </I18nProvider>
+      {/* QueryClientProvider must wrap ThemeProvider because the custom
+          ThemeProvider reads the admin-configured default via useQuery. */}
+      <QueryClientProvider>
+        <I18nProvider initialLocale={locale}>
+          <ThemeProvider>
+            <ChatProvider>{children}</ChatProvider>
+          </ThemeProvider>
+        </I18nProvider>
+      </QueryClientProvider>
     </NextThemesProvider>
   );
 }