feat: add transition effects for page and dropdown

- Add page transition CSS with opacity and blur effects

- Wrap dropdown in Transition component with slide effect

- Configure Nuxt pageTransition in config

frontend_nuxt/nuxt.config.ts

@@ -15,6 +15,7 @@ export default defineNuxtConfig({
   // 确保 Vditor 样式在 global.css 覆盖前加载
   css: ['vditor/dist/index.css', '~/assets/fonts.css', '~/assets/global.css'],
   app: {
+    pageTransition: { name: 'page', mode: 'out-in' },
     head: {
       script: [
         {