feat: 多主机纳管、用户认证、noVNC控制台、深色主题

主要功能:
- 多主机管理: 支持TCP/SSH方式纳管远程KVM主机
- 用户认证: JWT token认证, 默认admin/admin123
- noVNC控制台: 前端集成noVNC, WebSocket代理VNC连接
- 深色主题: 全局Element Plus深色主题覆盖
- 虚拟机操作: 克隆、迁移、XML编辑、快照管理
- 资源监控: CPU/内存/磁盘IO/网络流量实时监控

Bug修复:
- libvirt getInfo()内存单位修正(MiB非KiB)
- 远程主机VNC 0.0.0.0监听地址连接策略修复
- Dashboard定时器内存泄漏修复
- bcrypt版本兼容性修复

backend/app/auth.py

@@ -0,0 +1,102 @@
+"""用户认证模块 - JWT Token 认证"""
+from datetime import datetime, timedelta
+from typing import Optional
+from jose import JWTError, jwt
+from passlib.context import CryptContext
+from fastapi import Depends, HTTPException, status
+from fastapi.security import OAuth2PasswordBearer
+from pydantic import BaseModel
+from app.config import settings
+
+# 密码加密上下文
+pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
+
+# OAuth2 Token 提取
+oauth2_scheme = OAuth2PasswordBearer(tokenUrl=f"{settings.API_PREFIX}/auth/login")
+
+# 简易用户存储（生产环境应使用数据库）
+# 默认管理员账号: admin / admin123
+_users_db = {
+    "admin": {
+        "username": "admin",
+        "hashed_password": pwd_context.hash("admin123"),
+        "role": "admin",
+    }
+}
+
+
+class Token(BaseModel):
+    access_token: str
+    token_type: str = "bearer"
+    username: str = ""
+    role: str = ""
+
+
+class User(BaseModel):
+    username: str
+    role: str = "user"
+
+
+class UserCreate(BaseModel):
+    username: str
+    password: str
+    role: str = "user"
+
+
+class PasswordChange(BaseModel):
+    old_password: str
+    new_password: str
+
+
+def verify_password(plain_password: str, hashed_password: str) -> bool:
+    """验证密码"""
+    return pwd_context.verify(plain_password, hashed_password)
+
+
+def get_password_hash(password: str) -> str:
+    """生成密码哈希"""
+    return pwd_context.hash(password)
+
+
+def create_access_token(data: dict, expires_delta: Optional[timedelta] = None) -> str:
+    """创建 JWT Token"""
+    to_encode = data.copy()
+    expire = datetime.utcnow() + (expires_delta or timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES))
+    to_encode.update({"exp": expire})
+    return jwt.encode(to_encode, settings.SECRET_KEY, algorithm=settings.ALGORITHM)
+
+
+def authenticate_user(username: str, password: str) -> Optional[dict]:
+    """验证用户"""
+    user = _users_db.get(username)
+    if not user:
+        return None
+    if not verify_password(password, user["hashed_password"]):
+        return None
+    return user
+
+
+async def get_current_user(token: str = Depends(oauth2_scheme)) -> User:
+    """从 Token 获取当前用户（依赖注入）"""
+    credentials_exception = HTTPException(
+        status_code=status.HTTP_401_UNAUTHORIZED,
+        detail="无效的认证凭据",
+        headers={"WWW-Authenticate": "Bearer"},
+    )
+    try:
+        payload = jwt.decode(token, settings.SECRET_KEY, algorithms=[settings.ALGORITHM])
+        username: str = payload.get("sub")
+        if username is None:
+            raise credentials_exception
+    except JWTError:
+        raise credentials_exception
+
+    user = _users_db.get(username)
+    if user is None:
+        raise credentials_exception
+    return User(username=user["username"], role=user["role"])
+
+
+def get_users_db():
+    """获取用户数据库（用于路由中管理用户）"""
+    return _users_db