refactor: restructure passkey with dedicated package

- Create internal/passkey/ package with challenge session management
- Add HTTP handler layer in internal/api/passkey.go
- Simplify Passkey model to use blob credential storage
- Register passkey routes in router
- Update frontend to match new API endpoints
- Add .env.example template

backend/internal/dto/passkey.go

@@ -1,11 +0,0 @@
-package dto
-
-type Passkey struct {
-	ID         int64  `json:"id" gorm:"column:id;primaryKey;autoIncrement"`
-	Name       string `json:"name" gorm:"column:name"`                 // 凭证名称，用于用户识别不同的设备
-	SignCount  uint32 `json:"sign_count" gorm:"column:sign_count"`     // 签名计数器，用于防止重放攻击
-	DeviceType string `json:"device_type" gorm:"column:device_type"`   // 设备类型，如"platform"或"cross-platform"
-	LastUsedAt int64  `json:"last_used_at" gorm:"column:last_used_at"` // 最后使用时间
-	CreatedAt  int64  `json:"created_at,omitempty" gorm:"autoCreateTime"`
-	UpdatedAt  int64  `json:"updated_at,omitempty" gorm:"autoUpdateTime"`
-}