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/

go.mod

@@ -1,10 +1,20 @@
 module github.com/frenocorp/pop
 
-go 1.21
-
-require github.com/spf13/cobra v1.8.0
+go 1.23.0
 
 require (
-	github.com/inconshreveable/mousetrap v1.1.0 // indirect
-	github.com/spf13/pflag v1.0.5 // indirect
+	github.com/ProtonMail/gopenpgp/v2 v2.10.0
+	github.com/spf13/cobra v1.8.0
+)
+
+require (
+	github.com/ProtonMail/go-crypto v1.4.1 // indirect
+	github.com/ProtonMail/go-mime v0.0.0-20230322103455-7d82a3887f2f // indirect
+	github.com/cloudflare/circl v1.6.2 // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/pkg/errors v0.9.1 // indirect
+	github.com/spf13/pflag v1.0.5 // indirect
+	golang.org/x/crypto v0.41.0 // indirect
+	golang.org/x/sys v0.35.0 // indirect
+	golang.org/x/text v0.28.0 // indirect
 )