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/attachment/manager.go

@@ -0,0 +1,78 @@
+package attachment
+
+import (
+	"io"
+	"os"
+	"path/filepath"
+)
+
+type AttachmentManager struct {
+	attachmentsDir string
+}
+
+func NewAttachmentManager() *AttachmentManager {
+	return &AttachmentManager{
+		attachmentsDir: filepath.Join(os.Getenv("HOME"), ".config", "pop", "attachments"),
+	}
+}
+
+func (m *AttachmentManager) Download(attachmentID, name, destPath string) error {
+	if err := os.MkdirAll(m.attachmentsDir, 0755); err != nil {
+		return err
+	}
+
+	srcPath := filepath.Join(m.attachmentsDir, attachmentID)
+	dest := filepath.Join(destPath, name)
+
+	if err := os.MkdirAll(filepath.Dir(dest), 0755); err != nil {
+		return err
+	}
+
+	data, err := os.ReadFile(srcPath)
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(dest, data, 0644)
+}
+
+func (m *AttachmentManager) Upload(attachmentID, name string, reader io.Reader) error {
+	if err := os.MkdirAll(m.attachmentsDir, 0755); err != nil {
+		return err
+	}
+
+	path := filepath.Join(m.attachmentsDir, attachmentID)
+	
+	data, err := io.ReadAll(reader)
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(path, data, 0644)
+}
+
+func (m *AttachmentManager) Get(attachmentID string) ([]byte, error) {
+	path := filepath.Join(m.attachmentsDir, attachmentID)
+	return os.ReadFile(path)
+}
+
+func (m *AttachmentManager) Delete(attachmentID string) error {
+	path := filepath.Join(m.attachmentsDir, attachmentID)
+	return os.Remove(path)
+}
+
+func (m *AttachmentManager) List() ([]string, error) {
+	entries, err := os.ReadDir(m.attachmentsDir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return []string{}, nil
+		}
+		return nil, err
+	}
+
+	ids := make([]string, len(entries))
+	for i, entry := range entries {
+		ids[i] = entry.Name()
+	}
+	return ids, nil
+}