FRE-682: Add folder/label management, search, and fix PGP build

- Add pop mail search CLI command with pagination support
- Create internal/labels package with types and API client
- Add folder list/create/update/delete CLI commands
- Add label list/create/update/delete/apply/remove CLI commands
- Register folder and label commands in root.go
- Fix gopenpgp v2 API mismatches in pgp.go (NewPlainMessage, Armor,
  KeyRing.Encrypt/Decrypt, SessionKey)
- Fix NewSessionManager error handling across cmd files
- Fix variable shadowing bug in mail/client.go

cmd/root.go

@@ -24,6 +24,8 @@ func NewRootCmd() *cobra.Command {
 	rootCmd.AddCommand(mailDraftCmd())
 	rootCmd.AddCommand(contactCmd())
 	rootCmd.AddCommand(attachmentCmd())
+	rootCmd.AddCommand(folderCmd())
+	rootCmd.AddCommand(labelCmd())
 
 	return rootCmd
 }