初始提交：姜十三论坛 Jiang13 Forum

轻量自用论坛，Go 单二进制 + React SPA 内嵌 + SQLite。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/utils/theme.ts

@@ -0,0 +1,16 @@
+export type Theme = 'light' | 'dark';
+
+const STORAGE_KEY = 'j13-theme';
+
+/** 在 React 渲染前同步应用主题，避免首屏主题闪烁 */
+export function getStoredTheme(): Theme {
+  const stored = localStorage.getItem(STORAGE_KEY);
+  return stored === 'dark' ? 'dark' : 'light';
+}
+
+export function applyTheme(theme: Theme) {
+  const root = document.documentElement;
+  root.classList.toggle('dark', theme === 'dark');
+  root.style.colorScheme = theme;
+  localStorage.setItem(STORAGE_KEY, theme);
+}