feat: Implement internationalization with English and Thai locales and a language switcher.

Frontend-Learner/components/common/AppHeader.vue

@@ -26,18 +26,21 @@ const emit = defineEmits<{
     </div>
     
     <!-- Right Actions -->
-    <div class="flex items-center gap-4">
+    <div class="flex items-center gap-3">
       <!-- Search Bar (Optional) -->
       <div v-if="showSearch !== false" class="relative hidden-mobile" style="width: 300px;">
         <input 
           type="text" 
           class="input-field" 
-          placeholder="ค้นหาคอร์ส..." 
+          :placeholder="$t('menu.searchCourses')" 
           style="padding-left: 36px;"
         >
         <span style="position: absolute; left: 12px; top: 10px; color: var(--text-secondary);">🔍</span>
       </div>
       
+      <!-- Language Switcher (Left of Avatar) -->
+      <LanguageSwitcher />
+      
       <!-- User Profile Dropdown -->
       <UserMenu />
     </div>