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/config/config.go

@@ -61,7 +61,7 @@ func (m *ConfigManager) Load() (*Config, error) {
 }
 
 func (m *ConfigManager) Save(config *Config) error {
-	if err := os.MkdirAll(m.configDir, 0755); err != nil {
+	if err := os.MkdirAll(m.configDir, 0700); err != nil {
 		return fmt.Errorf("failed to create config dir: %w", err)
 	}