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/store/models.go

@@ -197,16 +197,8 @@ type Passkey struct {
 	ID           uint64    `gorm:"primaryKey;autoIncrement" json:"id"`
 	UserID       uint64    `gorm:"index;not null" json:"user_id"`
 	Name         string    `gorm:"size:64" json:"name"`
-	CredentialID string    `gorm:"size:255;not null" json:"-"`
-	PublicKey    string    `gorm:"size:512;not null" json:"-"`
-	AttestationType string `gorm:"size:64" json:"-"`
-	AAGUID       string    `gorm:"size:64" json:"-"`
-	SignCount    uint64    `json:"-"`
-	DeviceType   string    `gorm:"size:255" json:"device_type,omitempty"`
-	LastUsedAt   int64     `json:"last_used_at,omitempty"`
-	BackupEligible bool   `json:"-"`
-	BackupState    bool   `json:"-"`
-	Transport    string    `gorm:"size:32" json:"-"`
+	CredentialID []byte    `gorm:"size:255;not null" json:"-"`
+	Credential   []byte    `gorm:"type:blob;not null" json:"-"`
 	CreatedAt    time.Time `json:"created_at"`
 }