fix: 修正文件命名

- 与组件命名一致：button.variants.ts -> Button.variants.ts

src/lib/components/ui/button/Button.svelte

@@ -1,8 +1,8 @@
 <script lang="ts">
   import { Button as BitsButton, type WithoutChildren } from 'bits-ui';
   import type { Snippet } from 'svelte';
-  import { COLOR_MAP, VARIANT_MAP, SIZE_MAP } from './button.variants';
-  import type { Color, Variant, Size } from './button.variants';
+  import { COLOR_MAP, VARIANT_MAP, SIZE_MAP } from './Button.variants';
+  import type { Color, Variant, Size } from './Button.variants';
 
   type Props = WithoutChildren<BitsButton.RootProps> & {
     color?: Color;