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/mail.go

@@ -27,6 +27,7 @@ func mailCmd() *cobra.Command {
 	cmd.AddCommand(mailDeleteCmd())
 	cmd.AddCommand(mailTrashCmd())
 	cmd.AddCommand(mailDraftCmd())
+	cmd.AddCommand(mailSearchCmd())
 
 	return cmd
 }
@@ -46,7 +47,10 @@ func mailListCmd() *cobra.Command {
 				return fmt.Errorf("failed to load config: %w", err)
 			}
 
-			sessionMgr := auth.NewSessionManager()
+			sessionMgr, err := auth.NewSessionManager()
+			if err != nil {
+				return fmt.Errorf("failed to create session manager: %w", err)
+			}
 			session, err := sessionMgr.GetSession()
 			if err != nil {
 				return fmt.Errorf("not authenticated: %w", err)
@@ -141,7 +145,10 @@ func mailReadCmd() *cobra.Command {
 				return fmt.Errorf("failed to load config: %w", err)
 			}
 
-			sessionMgr := auth.NewSessionManager()
+			sessionMgr, err := auth.NewSessionManager()
+			if err != nil {
+				return fmt.Errorf("failed to create session manager: %w", err)
+			}
 			session, err := sessionMgr.GetSession()
 			if err != nil {
 				return fmt.Errorf("not authenticated: %w", err)
@@ -203,7 +210,10 @@ func mailSendCmd() *cobra.Command {
 				return fmt.Errorf("failed to load config: %w", err)
 			}
 
-			sessionMgr := auth.NewSessionManager()
+			sessionMgr, err := auth.NewSessionManager()
+			if err != nil {
+				return fmt.Errorf("failed to create session manager: %w", err)
+			}
 			session, err := sessionMgr.GetSession()
 			if err != nil {
 				return fmt.Errorf("not authenticated: %w", err)
@@ -259,7 +269,10 @@ func mailDeleteCmd() *cobra.Command {
 				return fmt.Errorf("failed to load config: %w", err)
 			}
 
-			sessionMgr := auth.NewSessionManager()
+			sessionMgr, err := auth.NewSessionManager()
+			if err != nil {
+				return fmt.Errorf("failed to create session manager: %w", err)
+			}
 			session, err := sessionMgr.GetSession()
 			if err != nil {
 				return fmt.Errorf("not authenticated: %w", err)
@@ -296,7 +309,10 @@ func mailTrashCmd() *cobra.Command {
 				return fmt.Errorf("failed to load config: %w", err)
 			}
 
-			sessionMgr := auth.NewSessionManager()
+			sessionMgr, err := auth.NewSessionManager()
+			if err != nil {
+				return fmt.Errorf("failed to create session manager: %w", err)
+			}
 			session, err := sessionMgr.GetSession()
 			if err != nil {
 				return fmt.Errorf("not authenticated: %w", err)
@@ -413,3 +429,80 @@ func formatSize(bytes int) string {
 		return fmt.Sprintf("%d B", bytes)
 	}
 }
+
+func mailSearchCmd() *cobra.Command {
+	var query, page, pageSize string
+
+	cmd := &cobra.Command{
+		Use:   "search <query>",
+		Short: "Search messages",
+		Long:  `Full-text search across messages in ProtonMail.`,
+		Args:  cobra.MaximumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			searchQuery := query
+			if len(args) > 0 && args[0] != "" {
+				searchQuery = args[0]
+			}
+			if searchQuery == "" {
+				return fmt.Errorf("search query is required")
+			}
+
+			cfgMgr := config.NewConfigManager()
+			cfg, err := cfgMgr.Load()
+			if err != nil {
+				return fmt.Errorf("failed to load config: %w", err)
+			}
+
+			sessionMgr, err := auth.NewSessionManager()
+			if err != nil {
+				return fmt.Errorf("failed to create session manager: %w", err)
+			}
+			session, err := sessionMgr.GetSession()
+			if err != nil {
+				return fmt.Errorf("not authenticated: %w", err)
+			}
+
+			client := api.NewProtonMailClient(cfg)
+			client.SetAuthHeader(session.AccessToken)
+			mailClient := mail.NewClient(client)
+
+			pageVal, err := strconv.Atoi(page)
+			if err != nil || pageVal < 1 {
+				pageVal = 1
+			}
+
+			pageSizeVal, err := strconv.Atoi(pageSize)
+			if err != nil || pageSizeVal < 1 {
+				pageSizeVal = 20
+			}
+			if pageSizeVal > 100 {
+				pageSizeVal = 100
+			}
+
+			req := mail.SearchRequest{
+				Query:      searchQuery,
+				Page:       pageVal,
+				PageSize:   pageSizeVal,
+				Passphrase: session.AccessToken,
+			}
+
+			result, err := mailClient.SearchMessages(req)
+			if err != nil {
+				return fmt.Errorf("failed to search messages: %w", err)
+			}
+
+			fmt.Printf("Found %d message(s) for query: %q\n", result.Total, searchQuery)
+			if len(result.Messages) == 0 {
+				return nil
+			}
+
+			return printMessages(result.Messages)
+		},
+	}
+
+	cmd.Flags().StringVar(&query, "query", "", "Search query (or pass as positional argument)")
+	cmd.Flags().StringVar(&page, "page", "1", "Page number")
+	cmd.Flags().StringVar(&pageSize, "page-size", "20", "Results per page (max 100)")
+
+	return cmd
+}