fix: 메인 헤더 원복 및 로그인 로고 크기 조정 (28px → 36px)

- main.html 헤더: SVG 아이콘+텍스트 방식으로 원복
- 로그인/접근거부 브랜드 패널 로고: 36px로 조정

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/templates/main.html

@@ -11,8 +11,19 @@
     <!-- ── 헤더 ── -->
     <header class="header">
       <div class="header-brand">
-        <img src="/static/dbx-logo.png" alt="DBX Corporation" class="header-logo-img" />
-        <span class="header-portal">업무 포털</span>
+        <div class="header-logo-mark">
+          <svg width="18" height="18" viewBox="0 0 24 24" fill="none"
+               stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round">
+            <rect x="3" y="3" width="7" height="7" rx="1"/>
+            <rect x="14" y="3" width="7" height="7" rx="1"/>
+            <rect x="3" y="14" width="7" height="7" rx="1"/>
+            <rect x="14" y="14" width="7" height="7" rx="1"/>
+          </svg>
+        </div>
+        <div class="header-brand-text">
+          <span class="header-company">DBX Corporation</span>
+          <span class="header-portal">업무 포털</span>
+        </div>
       </div>
 
       <div class="header-user">