feat(ui): implement dark mode with system preference support

- Add useDarkMode hook for managing theme state and persistence
- Integrate dark mode toggle button in app header
- Update all components with dark variant styles using Tailwind v4
- Create centralized style utilities for consistent theming
- Support system color scheme preference as fallback
- Store user preference in localStorage for persistence

src/lib/styles.ts

@@ -0,0 +1,68 @@
+/**
+ * 复用的 Tailwind 样式组合，覆盖常见 UI 模式
+ */
+
+// 按钮样式
+export const buttonStyles = {
+  // 主按钮：蓝底白字
+  primary: "px-4 py-2 bg-blue-500 text-white rounded-lg hover:bg-blue-600 dark:bg-blue-600 dark:hover:bg-blue-700 transition-colors text-sm font-medium",
+  
+  // 次按钮：灰背景，深色文本
+  secondary: "px-4 py-2 text-gray-500 hover:bg-gray-100 dark:text-gray-400 dark:hover:bg-gray-800 dark:hover:text-gray-200 rounded-lg transition-colors text-sm font-medium",
+  
+  // 危险按钮：用于不可撤销/破坏性操作
+  danger: "px-4 py-2 bg-red-500 text-white rounded-lg hover:bg-red-600 dark:bg-red-600 dark:hover:bg-red-700 transition-colors text-sm font-medium",
+  
+  // 幽灵按钮：无背景，仅悬浮反馈
+  ghost: "px-4 py-2 text-gray-500 hover:text-gray-900 hover:bg-gray-100 dark:text-gray-400 dark:hover:text-gray-100 dark:hover:bg-gray-800 rounded-lg transition-colors text-sm font-medium",
+  
+  // 图标按钮：小尺寸，仅图标
+  icon: "p-1.5 text-gray-500 hover:text-gray-900 hover:bg-gray-100 dark:text-gray-400 dark:hover:text-gray-100 dark:hover:bg-gray-800 rounded-md transition-colors",
+  
+  // 禁用态：可与其他样式组合
+  disabled: "opacity-50 cursor-not-allowed pointer-events-none",
+} as const;
+
+// 卡片样式
+export const cardStyles = {
+  // 基础卡片容器
+  base: "bg-white rounded-lg border border-gray-200 p-4 dark:bg-gray-900 dark:border-gray-700",
+  
+  // 带悬浮效果的卡片
+  interactive: "bg-white rounded-lg border border-gray-200 p-4 hover:border-gray-300 hover:shadow-sm dark:bg-gray-900 dark:border-gray-700 dark:hover:border-gray-600 transition-all duration-200",
+  
+  // 选中/激活态卡片
+  selected: "bg-white rounded-lg border border-blue-500 ring-1 ring-blue-500/20 bg-blue-500/5 p-4 dark:bg-gray-900 dark:border-blue-400 dark:ring-blue-400/20 dark:bg-blue-400/10",
+} as const;
+
+// 输入控件样式
+export const inputStyles = {
+  // 文本输入框
+  text: "w-full px-3 py-2 border border-gray-200 rounded-lg focus:border-blue-500 focus:ring-1 focus:ring-blue-500/20 outline-none dark:bg-gray-900 dark:border-gray-700 dark:text-gray-100 dark:focus:border-blue-400 dark:focus:ring-blue-400/20 transition-colors",
+  
+  // 下拉选择框
+  select: "w-full px-3 py-2 border border-gray-200 rounded-lg focus:border-blue-500 focus:ring-1 focus:ring-blue-500/20 outline-none bg-white dark:bg-gray-900 dark:border-gray-700 dark:text-gray-100 dark:focus:border-blue-400 dark:focus:ring-blue-400/20 transition-colors",
+  
+  // 复选框
+  checkbox: "w-4 h-4 text-blue-500 rounded focus:ring-blue-500/20 border-gray-300 dark:border-gray-600 dark:bg-gray-800",
+} as const;
+
+// 徽标（Badge）样式
+export const badgeStyles = {
+  // 成功徽标
+  success: "inline-flex items-center gap-1 px-2 py-1 bg-green-500/10 text-green-500 rounded-md text-xs font-medium",
+  
+  // 信息徽标
+  info: "inline-flex items-center gap-1 px-2 py-1 bg-blue-500/10 text-blue-500 rounded-md text-xs font-medium",
+  
+  // 警告徽标
+  warning: "inline-flex items-center gap-1 px-2 py-1 bg-amber-500/10 text-amber-500 rounded-md text-xs font-medium",
+  
+  // 错误徽标
+  error: "inline-flex items-center gap-1 px-2 py-1 bg-red-500/10 text-red-500 rounded-md text-xs font-medium",
+} as const;
+
+// 组合类名的工具函数
+export function cn(...classes: (string | undefined | false)[]) {
+  return classes.filter(Boolean).join(' ');
+}