fix: theme.ts 所有函数添加 SSR 守卫，校验 localStorage 合法值

- setStoredTheme/applyTheme/initTheme 添加 typeof window 检查
- getStoredTheme 校验存储值是否在白名单中，防止非法 theme 值

src/lib/theme.ts

@@ -1,13 +1,18 @@
 export type Theme = "light" | "dark" | "system";
 
 const STORAGE_KEY = "nowhatever-theme";
+const VALID_THEMES: Theme[] = ["light", "dark", "system"];
 
 export function getStoredTheme(): Theme {
   if (typeof window === "undefined") return "system";
-  return (localStorage.getItem(STORAGE_KEY) as Theme) || "system";
+  const stored = localStorage.getItem(STORAGE_KEY);
+  return stored && VALID_THEMES.includes(stored as Theme)
+    ? (stored as Theme)
+    : "system";
 }
 
 export function setStoredTheme(theme: Theme): void {
+  if (typeof window === "undefined") return;
   localStorage.setItem(STORAGE_KEY, theme);
   applyTheme(theme);
 }
@@ -21,11 +26,13 @@ function resolveTheme(theme: Theme): "light" | "dark" {
 }
 
 export function applyTheme(theme: Theme): void {
+  if (typeof window === "undefined") return;
   const resolved = resolveTheme(theme);
   document.documentElement.setAttribute("data-theme", resolved);
 }
 
 export function initTheme(): void {
+  if (typeof window === "undefined") return;
   const theme = getStoredTheme();
   applyTheme(theme);