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/

cmd/root.go

@@ -20,6 +20,8 @@ func NewRootCmd() *cobra.Command {
 	rootCmd.AddCommand(loginCmd())
 	rootCmd.AddCommand(logoutCmd())
 	rootCmd.AddCommand(sessionCmd())
+	rootCmd.AddCommand(contactCmd())
+	rootCmd.AddCommand(attachmentCmd())
 
 	return rootCmd
 }