feat: 全局主题切换（浅色/深色/跟随系统）

- CSS 变量驱动的主题系统，所有颜色响应 data-theme 属性
- 新增语义化色彩 heading/secondary/tertiary，替换硬编码 text-white/text-gray-*
- 右上角三态主题按钮（自动/浅色/深色），全局可用无需登录
- layout.tsx 内联脚本防闪烁
- 修复个人中心页面溢出无法滚动

src/lib/theme.ts

@@ -0,0 +1,37 @@
+export type Theme = "light" | "dark" | "system";
+
+const STORAGE_KEY = "nowhatever-theme";
+
+export function getStoredTheme(): Theme {
+  if (typeof window === "undefined") return "system";
+  return (localStorage.getItem(STORAGE_KEY) as Theme) || "system";
+}
+
+export function setStoredTheme(theme: Theme): void {
+  localStorage.setItem(STORAGE_KEY, theme);
+  applyTheme(theme);
+}
+
+function resolveTheme(theme: Theme): "light" | "dark" {
+  if (theme !== "system") return theme;
+  if (typeof window === "undefined") return "dark";
+  return window.matchMedia("(prefers-color-scheme: light)").matches
+    ? "light"
+    : "dark";
+}
+
+export function applyTheme(theme: Theme): void {
+  const resolved = resolveTheme(theme);
+  document.documentElement.setAttribute("data-theme", resolved);
+}
+
+export function initTheme(): void {
+  const theme = getStoredTheme();
+  applyTheme(theme);
+
+  if (theme === "system") {
+    window
+      .matchMedia("(prefers-color-scheme: light)")
+      .addEventListener("change", () => applyTheme("system"));
+  }
+}