feat(frontend): add theme real-time sync feature

1. 新增ThemeSync组件实现站点主题色无刷新同步
2. 给获取站点设置的接口添加no-store缓存策略确保拿到最新配置
3. 调整主题应用代码的属性设置顺序避免逻辑问题

frontend/lib/theme.ts

@@ -236,8 +236,8 @@ export function applyAccentTheme(input: string | null): void {
   if (!el) {
     el = document.createElement("style");
     el.id = STYLE_ELEMENT_ID;
-    el.setAttribute("data-accent-source", "runtime");
     document.head.appendChild(el);
   }
+  el.setAttribute("data-accent-source", "runtime");
   el.textContent = css;
 }