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/mail/client.go

@@ -238,7 +238,7 @@ func (c *Client) SaveDraft(draft Draft, passphrase string) (string, error) {
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(resp.Body)
+	respBody, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return "", fmt.Errorf("failed to read response: %w", err)
 	}
@@ -248,7 +248,7 @@ func (c *Client) SaveDraft(draft Draft, passphrase string) (string, error) {
 			MessageID string `json:"MessageID"`
 		} `json:"Data"`
 	}
-	if err := json.Unmarshal(body, &result); err != nil {
+	if err := json.Unmarshal(respBody, &result); err != nil {
 		return "", fmt.Errorf("failed to parse response: %w", err)
 	}