FRE-683: Add contacts & attachments management

- Implemented contact CRUD operations (list, add, edit, delete)
- Implemented attachment management (list, upload, download)
- Created internal/contact/manager.go for contact persistence
- Created internal/attachment/manager.go for attachment storage
- Added CLI commands in cmd/contacts.go and cmd/attachments.go
- Integrated contact and attachment commands into root CLI

Files:
- internal/contact/types.go - Contact data models
- internal/contact/manager.go - Contact CRUD operations
- internal/attachment/manager.go - Attachment file operations
- cmd/contacts.go - Contact CLI commands
- cmd/attachments.go - Attachment CLI commands

Contacts stored in ~/.config/pop/contacts.json
Attachments stored in ~/.config/pop/attachments/

internal/contact/types.go

@@ -0,0 +1,54 @@
+package contact
+
+import "time"
+
+type Contact struct {
+	ID          string    `json:"ID"`
+	Email       string    `json:"Email"`
+	Name        string    `json:"Name,omitempty"`
+	Phone       string    `json:"Phone,omitempty"`
+	Address     string    `json:"Address,omitempty"`
+	Notes       string    `json:"Notes,omitempty"`
+	IsFavorite  bool      `json:"IsFavorite,omitempty"`
+	CustomFields map[string]string `json:"CustomFields,omitempty"`
+	CreatedAt   time.Time `json:"CreatedAt,omitempty"`
+	UpdatedAt   time.Time `json:"UpdatedAt,omitempty"`
+}
+
+type ContactGroup struct {
+	ID       string   `json:"ID"`
+	Name     string   `json:"Name"`
+	ContactIDs []string `json:"ContactIDs,omitempty"`
+}
+
+type ListContactsRequest struct {
+	Page       int    `json:"Page"`
+	PageSize   int    `json:"PageSize"`
+	Search     string `json:"Search,omitempty"`
+	IsFavorite *bool  `json:"IsFavorite,omitempty"`
+}
+
+type ListContactsResponse struct {
+	Total   int        `json:"Total"`
+	Contacts []Contact `json:"Contacts"`
+}
+
+type CreateContactRequest struct {
+	Email        string            `json:"Email"`
+	Name         string            `json:"Name,omitempty"`
+	Phone        string            `json:"Phone,omitempty"`
+	Address      string            `json:"Address,omitempty"`
+	Notes        string            `json:"Notes,omitempty"`
+	IsFavorite   bool              `json:"IsFavorite,omitempty"`
+	CustomFields map[string]string `json:"CustomFields,omitempty"`
+}
+
+type UpdateContactRequest struct {
+	Email        *string            `json:"Email,omitempty"`
+	Name         *string            `json:"Name,omitempty"`
+	Phone        *string            `json:"Phone,omitempty"`
+	Address      *string            `json:"Address,omitempty"`
+	Notes        *string            `json:"Notes,omitempty"`
+	IsFavorite   *bool              `json:"IsFavorite,omitempty"`
+	CustomFields map[string]string  `json:"CustomFields,omitempty"`
+}