fix: replace vue3-flag-icons with custom CSS-only flag icons

- Remove vue3-flag-icons dependency (was causing slow loading)
- Create custom flag-icons.css with only cn and us flags (inline SVG)
- Update language-change.vue to use CSS-based flag icons
- Delete vue3-flag-icons type declaration file
- Flag icons now load instantly without any JavaScript overhead

frontend/src/components/language-change.vue

@@ -3,7 +3,6 @@ import type { AcceptableValue } from 'reka-ui'
 
 import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
-import FlagIcon from 'vue3-flag-icons'
 
 import type { Language } from '@/plugins/i18n'
 
@@ -35,8 +34,12 @@ const currentFlagCode = computed(() => {
   <UiDropdownMenu>
     <UiDropdownMenuTrigger as-child>
       <UiButton variant="ghost" size="icon" class="h-9 w-9">
-        <FlagIcon :code="currentFlagCode" :size="20" />
-        <span class="sr-only">切换语言</span>
+        <span
+          class="flag-icon"
+          :class="`flag-icon-${currentFlagCode}`"
+          :style="{ width: '20px', height: '15px' }"
+        />
+        <span class="sr-only">{{ $t('changeLanguage') }}</span>
       </UiButton>
     </UiDropdownMenuTrigger>
     <UiDropdownMenuContent align="end">
@@ -47,11 +50,11 @@ const currentFlagCode = computed(() => {
         @update:model-value="handleLocaleChange"
       >
         <UiDropdownMenuRadioItem value="en">
-          <FlagIcon code="us" :size="18" />
+          <span class="flag-icon flag-icon-us" :style="{ width: '18px', height: '13.5px' }" />
           <span class="ml-2">English</span>
         </UiDropdownMenuRadioItem>
         <UiDropdownMenuRadioItem value="zh">
-          <FlagIcon code="cn" :size="18" />
+          <span class="flag-icon flag-icon-cn" :style="{ width: '18px', height: '13.5px' }" />
           <span class="ml-2">中文</span>
         </UiDropdownMenuRadioItem>
       </UiDropdownMenuRadioGroup>