fix: 修复输入框和按钮在暗色模式下的样式问题

- 为 :root 和 .dark 添加 color-scheme，使浏览器表单控件默认样式适配暗色模式
- 将 bg-primary 搭配 text-white 的按钮改为 text-primary-foreground
- 替换残留的 hover:bg-blue-700、text-blue-500、bg-gray-200 等硬编码颜色
- 同步更新 TextInputArea 单元测试

pages/Timestamp/LiveClock.tsx

@@ -47,7 +47,7 @@ const LiveClock = React.memo(({ unit, onUseNow }: LiveClockProps) => {
       <button
         onClick={handleUseNow}
         title={t('timestamp:useNowTooltip')}
-        className="p-1.5 rounded-md bg-background text-primary shadow-sm hover:bg-blue-700 hover:text-white transition-colors"
+        className="p-1.5 rounded-md bg-background text-primary shadow-sm hover:bg-primary hover:text-primary-foreground transition-colors"
       >
         <Clock className="w-4 h-4" />
       </button>

pages/Timestamp/index.tsx

@@ -98,7 +98,7 @@ export default function Index() {
             {/* 立即转换 */}
             <button
               onClick={convert}
-              className="block mx-auto mt-4 mb-1 max-w-[240px] w-full py-2.5 text-sm font-semibold rounded-lg bg-primary text-white hover:bg-blue-700 focus:outline-none focus:ring-2 focus:ring-primary focus:ring-offset-2 transition-all"
+              className="block mx-auto mt-4 mb-1 max-w-[240px] w-full py-2.5 text-sm font-semibold rounded-lg bg-primary text-primary-foreground hover:bg-primary/90 focus:outline-none focus:ring-2 focus:ring-primary focus:ring-offset-2 transition-all"
             >
               {t('timestamp:convertButton')}
             </button>