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

backend/python/app/api/admin/settings.py

@@ -267,6 +267,77 @@ async def update_sms_config(
     return {"message": "短信配置已更新"}
 
 
+
+# ============================================================
+# Logo 上传
+# ============================================================
+
+from fastapi import UploadFile, File
+
+@router.post("/logo/upload")
+async def upload_logo(
+    file: UploadFile = File(...),
+    admin_user=Depends(get_current_admin),
+    db: Session = Depends(get_db),
+):
+    """上传平台Logo，保存到本地data目录，返回访问URL"""
+    import os as _os
+    import shutil as _shutil
+
+    # Validate file type
+    allowed_types = ['image/png', 'image/jpeg', 'image/svg+xml', 'image/webp']
+    if file.content_type not in allowed_types:
+        raise HTTPException(status_code=400, detail=f"不支持的文件类型: {file.content_type}，仅支持 PNG/JPG/SVG/WEBP")
+
+    # Validate size (max 2MB)
+    contents = await file.read()
+    if len(contents) > 2 * 1024 * 1024:
+        raise HTTPException(status_code=400, detail="文件大小不能超过 2MB")
+
+    # Save to /app/data/uploads/logos/
+    upload_dir = "/app/data/uploads/logos"
+    _os.makedirs(upload_dir, exist_ok=True)
+
+    # Generate unique filename
+    ext = file.filename.split('.')[-1] if '.' in (file.filename or '') else 'png'
+    filename = f"{uuid.uuid4()}.{ext}"
+    filepath = _os.path.join(upload_dir, filename)
+
+    with open(filepath, 'wb') as f:
+        f.write(contents)
+
+    # Build URL
+    logo_url = f"/data/uploads/logos/{filename}"
+
+    # Update site setting
+    existing = db.query(SiteSetting).filter(SiteSetting.key == "logo").first()
+    if existing:
+        existing.value = logo_url
+        existing.updated_at = datetime.now()
+    else:
+        new_setting = SiteSetting(
+            id=str(uuid.uuid4()),
+            key="logo",
+            value=logo_url,
+        )
+        db.add(new_setting)
+    db.commit()
+
+    LogService.log_audit(
+        admin_id=admin_user.id,
+        admin_name=admin_user.username,
+        action="upload_logo",
+        target_type="site_settings",
+        detail=f"上传Logo: {filename}",
+    )
+
+    return {
+        "code": 0,
+        "data": {"url": logo_url, "filename": filename},
+        "message": "Logo 上传成功",
+    }
+
+
 # ============================================================
 # 邮件测试
 # ============================================================