Phase C3: Site Settings Refactoring with horizontal sub-tabs and Logo upload

- Replaced vertical card layout with 5 horizontal tabs: 基础设置/Logo设置/邮件配置/短信配置/注册设置
- Added Logo upload tab with drag-drop preview and file upload
- Added Registration Settings tab (allow_register, email/phone verify, default role, welcome message)
- Added logo upload backend endpoint (POST /admin/settings/logo/upload)
- Added modal sub-component CSS classes for dark theme
- Fixed all hardcoded white backgrounds for dark theme
- Added uploadLogo method to adminAPI

frontend/src/components/layout/AdminLayout.css

@@ -896,6 +896,66 @@
 }
 
 /* ========== Responsive ========== */
+
+
+/* Modal sub-components */
+.admin-modal-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 20px;
+  padding-bottom: 16px;
+  border-bottom: 1px solid rgba(255, 255, 255, 0.06);
+}
+
+.admin-modal-header h3 {
+  margin: 0 !important;
+}
+
+.admin-modal-close {
+  background: none;
+  border: none;
+  color: #8892B0;
+  cursor: pointer;
+  padding: 4px;
+  border-radius: 6px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition: all 0.2s;
+}
+
+.admin-modal-close:hover {
+  color: #E8ECF4;
+  background: rgba(255, 255, 255, 0.06);
+}
+
+.admin-modal-body {
+  margin-bottom: 20px;
+}
+
+.admin-modal-footer {
+  display: flex;
+  gap: 8px;
+  justify-content: flex-end;
+  padding-top: 16px;
+  border-top: 1px solid rgba(255, 255, 255, 0.06);
+}
+
+/* Spinner animation for logo upload */
+.spinner {
+  width: 32px;
+  height: 32px;
+  border: 3px solid rgba(108, 99, 255, 0.2);
+  border-top-color: #6C63FF;
+  border-radius: 50%;
+  animation: spin 0.8s linear infinite;
+}
+
+@keyframes spin {
+  to { transform: rotate(360deg); }
+}
+
 @media (max-width: 768px) {
   .admin-sidebar {
     position: fixed;