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/core/system-settings/api.ts

@@ -65,6 +65,48 @@ export async function loadPrivateSquareStatus(): Promise<PrivateSquareStatus> {
   return (await response.json()) as PrivateSquareStatus;
 }
 
+export type ThemeMode = "light" | "dark";
+
+export interface AppearanceSettings {
+  default_theme: ThemeMode;
+}
+
+export interface AppearanceUpdate {
+  default_theme?: ThemeMode;
+}
+
+export async function loadAppearanceSettings(): Promise<AppearanceSettings> {
+  const response = await apiFetch(
+    `${getBackendBaseURL()}/api/system-settings/appearance`,
+  );
+  if (!response.ok) {
+    throw new Error(
+      `Failed to load appearance settings: ${response.status}`,
+    );
+  }
+  return (await response.json()) as AppearanceSettings;
+}
+
+export async function updateAppearanceSettings(
+  body: AppearanceUpdate,
+): Promise<AppearanceSettings> {
+  const response = await apiFetch(
+    `${getBackendBaseURL()}/api/system-settings/appearance`,
+    {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify(body),
+    },
+  );
+  if (!response.ok) {
+    const detail = await response
+      .json()
+      .catch(() => ({ detail: "Failed to update appearance" }));
+    throw new Error(detail.detail ?? "Failed to update appearance");
+  }
+  return (await response.json()) as AppearanceSettings;
+}
+
 export async function updatePrivateSquareSettings(
   body: PrivateSquareUpdate,
 ): Promise<PrivateSquareStatus> {