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

internal/labels/types.go

@@ -0,0 +1,58 @@
+package labels
+
+// Folder represents a ProtonMail folder (system or custom)
+type Folder struct {
+	ID           string `json:"ID"`
+	Name         string `json:"Name"`
+	Type         int    `json:"Type"`
+	MessageCount int    `json:"MessageCount,omitempty"`
+	ParentID     string `json:"ParentID,omitempty"`
+	SortOrder    int    `json:"SortOrder,omitempty"`
+}
+
+// Label represents a user-created label/tag
+type Label struct {
+	ID   string `json:"ID"`
+	Name string `json:"Name"`
+	Color string `json:"Color,omitempty"`
+}
+
+// CreateFolderRequest for creating a new folder
+type CreateFolderRequest struct {
+	Name     string `json:"Name"`
+	ParentID string `json:"ParentID,omitempty"`
+}
+
+// UpdateFolderRequest for updating a folder
+type UpdateFolderRequest struct {
+	Name      string `json:"Name,omitempty"`
+	SortOrder *int   `json:"SortOrder,omitempty"`
+}
+
+// CreateLabelRequest for creating a new label
+type CreateLabelRequest struct {
+	Name  string `json:"Name"`
+	Color string `json:"Color,omitempty"`
+}
+
+// UpdateLabelRequest for updating a label
+type UpdateLabelRequest struct {
+	Name  string  `json:"Name,omitempty"`
+	Color *string `json:"Color,omitempty"`
+}
+
+// LabelMessageRequest for applying/removing labels from messages
+type LabelMessageRequest struct {
+	MessageID string `json:"MessageID"`
+	LabelID   string `json:"LabelID"`
+}
+
+// ListFoldersResponse for listing folders
+type ListFoldersResponse struct {
+	Folders []Folder `json:"Folders"`
+}
+
+// ListLabelsResponse for listing labels
+type ListLabelsResponse struct {
+	Labels []Label `json:"Labels"`
+}