fix: 사용자 이름 첫 글자 한글 깨짐 수정

- substr() → mb_substr()로 변경하여 멀티바이트 문자 지원
- strtoupper() → mb_strtoupper()로 변경
- 헤더, 사이드바 사용자 아바타에 적용

resources/views/partials/header.blade.php

@@ -70,7 +70,7 @@ class="border-gray-300 rounded-lg text-sm focus:ring-primary focus:border-primar
                 class="flex items-center gap-2 px-3 py-2 text-sm font-medium text-gray-700 hover:bg-gray-100 rounded-lg focus:outline-none"
             >
                 <div class="w-8 h-8 rounded-full bg-primary text-white flex items-center justify-center text-sm font-bold">
-                    {{ strtoupper(substr(auth()->user()->name ?? 'U', 0, 1)) }}
+                    {{ mb_strtoupper(mb_substr(auth()->user()->name ?? 'U', 0, 1)) }}
                 </div>
                 <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                     <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M19 9l-7 7-7-7" />

resources/views/partials/sidebar.blade.php

@@ -331,7 +331,7 @@ class="flex items-center gap-2 pr-3 py-2 rounded-lg text-sm text-gray-700 hover:
         <div class="p-4 border-t border-gray-200">
             <div class="flex items-center gap-3">
                 <div class="w-10 h-10 rounded-full bg-primary text-white flex items-center justify-center font-bold">
-                    {{ strtoupper(substr(auth()->user()->name ?? 'U', 0, 1)) }}
+                    {{ mb_strtoupper(mb_substr(auth()->user()->name ?? 'U', 0, 1)) }}
                 </div>
                 <div class="flex-1 min-w-0">
                     <p class="text-sm font-medium text-gray-900 truncate">