feat(ui): Implement Dropdown Menu components for language switcher

- Add DropdownMenu, DropdownMenuTrigger, DropdownMenuContent, and DropdownMenuItem components
- Refactor LanguageSwitcher to use new Dropdown Menu components
- Update Navbar to use LanguageSwitcher component with improved language selection UI
- Remove legacy language toggle logic from Navbar

src/components/ui/dropdown-menu/DropdownMenu.vue

@@ -0,0 +1,17 @@
+<script setup lang="ts">
+import { DropdownMenuRoot } from 'radix-vue'
+
+defineOptions({
+  name: 'DropdownMenu'
+})
+
+defineSlots<{
+  default?: (props: {}) => any
+}>()
+</script>
+
+<template>
+  <DropdownMenuRoot v-bind="$attrs">
+    <slot />
+  </DropdownMenuRoot>
+</template>