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/Base64Converter/FileMode.tsx

@@ -195,7 +195,7 @@ export default function FileMode() {
               }}
             />
             {isLoading ? (
-              <div className="w-10 h-10 border-4 border-primary/30 border-t-blue-600 rounded-full animate-spin" />
+              <div className="w-10 h-10 border-4 border-primary/30 border-t-primary rounded-full animate-spin" />
             ) : info ? (
               <div className="flex flex-col items-center gap-1">
                 <Upload className="w-10 h-10 text-primary" />

pages/Base64Converter/ImageMode.tsx

@@ -202,7 +202,7 @@ export default function ImageMode() {
               }}
             />
             {isLoading ? (
-              <div className="w-10 h-10 border-4 border-primary/30 border-t-blue-600 rounded-full animate-spin" />
+              <div className="w-10 h-10 border-4 border-primary/30 border-t-primary rounded-full animate-spin" />
             ) : info ? (
               <div className="flex flex-col items-center gap-1">
                 {result && (