fix(ui): improve background handling and PWA update prompts

- Add adaptive solid color background support for dark/light themes.
- Replace mesh gradient with semi-transparent blurred background images.
- Fix background scrolling issue on mobile browsers.
- Remove borders from specific cards for consistent UI styling.
- Prevent caching of PWA service worker files in Nginx.
- Introduce interactive toast prompt for PWA updates instead of silent refresh.

vite-frontend/src/components/theme-settings.tsx

@@ -54,7 +54,7 @@ export const ThemeSettings: React.FC = () => {
   };
 
   return (
-    <Card className="border border-gray-200 dark:border-gray-700">
+    <Card className="shadow-md">
       <CardBody className="p-6">
         <h2 className="text-lg font-medium text-gray-900 dark:text-white mb-5">
           主题设置