Improve startup logging and logging consistency

Startup log:
- Add interface, port, and fileTransferPort fields to the "Hotline
  server started" line so operators can see what the server bound to.
- Resolve an empty -interface flag to 0.0.0.0 for display.

Levels:
- Demote the two Redis startup messages (ban management, cleared online
  users) from Info to Debug.

Consistency:
- Standardize the error field key to "err" (was "Err" in a few account
  handlers) and lowercase "Account" -> "account".
- Standardize the remote-address key to "remoteAddr" (was "RemoteAddr").
- Replace fmt.Sprintf in the tracker-registration message and string
  concatenation in the config-dir-init and ban-disconnect messages with
  structured fields; use the standard "err" key.
- Give the two bare rLogger.Error(err.Error()) file-transfer calls a
  descriptive message and an "err" field.

logger.go:
- Only attach the rotating lumberjack file writer when --log-file is set.
  An empty Filename made lumberjack write to a temp file by default.

7 files changed, 34 insertions, 23 deletions

diff --git a/cmd/mobius-hotline-server/main.go b/cmd/mobius-hotline-server/main.go
index 8b9d54c..dced2c5 100644
--- a/cmd/mobius-hotline-server/main.go
+++ b/cmd/mobius-hotline-server/main.go
@@ -71,7 +71,7 @@ func main() {
 				slogger.Error("Error copying config dir", "err", err)
 				os.Exit(1)
 			}
-			slogger.Info("Config dir initialized at " + *configDir)
+			slogger.Info("Config dir initialized", "config", *configDir)
 		} else {
 			slogger.Info("Existing config dir found.  Skipping initialization.")
 		}
@@ -131,7 +131,7 @@ func main() {
 
 		srv.Redis = redisClient
 		srv.BanList = mobius.NewRedisBanMgr(redisClient, slogger)
-		slogger.Info("Using Redis for ban management", "addr", *redisAddr)
+		slogger.Debug("Using Redis for ban management", "addr", *redisAddr)
 	} else {
 		srv.BanList, err = mobius.NewBanFile(path.Join(*configDir, "Banlist.yaml"))
 		if err != nil {
@@ -215,7 +215,11 @@ func main() {
 		}
 	}()
 
-	slogger.Info("Hotline server started", "version", version, "config", *configDir)
+	boundInterface := *netInterface
+	if boundInterface == "" {
+		boundInterface = "0.0.0.0"
+	}
+	slogger.Info("Hotline server started", "version", version, "config", *configDir, "interface", boundInterface, "port", *basePort, "fileTransferPort", *basePort+1)
 	if tlsConfig != nil {
 		slogger.Info("TLS enabled", "port", *tlsPort, "fileTransferPort", *tlsPort+1)
 	}
diff --git a/hotline/client_conn.go b/hotline/client_conn.go
index 9ca93a7..c17ef35 100644
--- a/hotline/client_conn.go
+++ b/hotline/client_conn.go
@@ -176,7 +176,7 @@ func (cc *ClientConn) Disconnect() {
 	}
 
 	if err := cc.Connection.Close(); err != nil {
-		cc.Server.Logger.Debug("error closing client connection", "RemoteAddr", cc.RemoteAddr)
+		cc.Server.Logger.Debug("error closing client connection", "remoteAddr", cc.RemoteAddr)
 	}
 }
 
diff --git a/hotline/file_transfer.go b/hotline/file_transfer.go
index 98999f1..86e10eb 100644
--- a/hotline/file_transfer.go
+++ b/hotline/file_transfer.go
@@ -477,7 +477,7 @@ func DownloadFolderHandler(rwc io.ReadWriter, fullPath string, fileTransfer *Fil
 
 		// Send file size to client
 		if _, err := rwc.Write(hlFile.Ffo.TransferSize(dataOffset)); err != nil {
-			rLogger.Error(err.Error())
+			rLogger.Error("Error sending file size", "err", err)
 			return fmt.Errorf("error sending file size: %w", err)
 		}
 
@@ -625,7 +625,7 @@ func UploadFolderHandler(rwc io.ReadWriter, fullPath string, fileTransfer *FileT
 				}
 
 				if err := receiveFile(rwc, file, io.Discard, io.Discard, fileTransfer.bytesSentCounter); err != nil {
-					rLogger.Error(err.Error())
+					rLogger.Error("Error receiving file", "err", err)
 				}
 
 				err = os.Rename(fullPath+"/"+fu.FormattedPath()+".incomplete", fullPath+"/"+fu.FormattedPath())
diff --git a/hotline/server.go b/hotline/server.go
index 7bc6a4a..6ac68d8 100644
--- a/hotline/server.go
+++ b/hotline/server.go
@@ -309,16 +309,16 @@ func (s *Server) Serve(ctx context.Context, ln net.Listener) error {
 
 				// Check if the rate limit is exceeded and close the connection if so.
 				if !rl.Allow() {
-					s.Logger.Info("Rate limit exceeded", "RemoteAddr", conn.RemoteAddr())
+					s.Logger.Info("Rate limit exceeded", "remoteAddr", conn.RemoteAddr())
 					_ = conn.Close()
 					return
 				}
 
 				if err := s.handleNewConnection(connCtx, conn, conn.RemoteAddr().String()); err != nil {
 					if err == io.EOF {
-						s.Logger.Info("Client disconnected", "RemoteAddr", conn.RemoteAddr())
+						s.Logger.Info("Client disconnected", "remoteAddr", conn.RemoteAddr())
 					} else {
-						s.Logger.Error("Error serving request", "RemoteAddr", conn.RemoteAddr(), "err", err)
+						s.Logger.Error("Error serving request", "remoteAddr", conn.RemoteAddr(), "err", err)
 					}
 				}
 			}()
@@ -380,7 +380,7 @@ func (s *Server) registerWithAllTrackers() {
 		tr.Password = parseTrackerPassword(t)
 
 		if err := s.TrackerRegistrar.Register(t, tr); err != nil {
-			s.Logger.Error(fmt.Sprintf("Unable to register with tracker %v", t), "error", err)
+			s.Logger.Error("Unable to register with tracker", "tracker", t, "err", err)
 		}
 	}
 }
diff --git a/internal/mobius/api.go b/internal/mobius/api.go
index adc9072..a574bf8 100644
--- a/internal/mobius/api.go
+++ b/internal/mobius/api.go
@@ -85,7 +85,7 @@ func NewAPIServer(hlServer *hotline.Server, reloadFunc func(), logger *slog.Logg
 		if err := hlServer.Redis.Del(context.Background(), hotline.RedisKeyOnline).Err(); err != nil {
 			srv.logger.Warn("Failed to clear online users in Redis", "err", err)
 		} else {
-			srv.logger.Info("Cleared online users in Redis on startup")
+			srv.logger.Debug("Cleared online users in Redis on startup")
 		}
 	}
 
diff --git a/internal/mobius/logger.go b/internal/mobius/logger.go
index 60d4f0d..2f67a71 100644
--- a/internal/mobius/logger.go
+++ b/internal/mobius/logger.go
@@ -22,14 +22,21 @@ var logLevels = map[string]slog.Level{
 }
 
 func NewLogger(logLevel, logFile *string) *slog.Logger {
+	// Always log to stdout. Only add the rotating file writer when a log file path
+	// is configured; an empty Filename makes lumberjack write to a temp file.
+	var out io.Writer = os.Stdout
+	if *logFile != "" {
+		out = io.MultiWriter(os.Stdout, &lumberjack.Logger{
+			Filename:   *logFile,
+			MaxSize:    logMaxSize,
+			MaxBackups: logMaxBackups,
+			MaxAge:     logMaxAge,
+		})
+	}
+
 	return slog.New(
 		slog.NewTextHandler(
-			io.MultiWriter(os.Stdout, &lumberjack.Logger{
-				Filename:   *logFile,
-				MaxSize:    logMaxSize,
-				MaxBackups: logMaxBackups,
-				MaxAge:     logMaxAge,
-			}),
+			out,
 			&slog.HandlerOptions{
 				Level: logLevels[*logLevel],
 				ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
diff --git a/internal/mobius/transaction_handlers.go b/internal/mobius/transaction_handlers.go
index b629be8..96b4f69 100644
--- a/internal/mobius/transaction_handlers.go
+++ b/internal/mobius/transaction_handlers.go
@@ -685,7 +685,7 @@ func HandleSetUser(cc *hotline.ClientConn, t *hotline.Transaction) (res []hotlin
 
 	err := cc.Server.AccountManager.Update(*account, account.Login)
 	if err != nil {
-		cc.Logger.Error("Error updating account", "Err", err)
+		cc.Logger.Error("Error updating account", "err", err)
 		return cc.NewErrReply(t, ErrMsgUpdateAccount)
 	}
 
@@ -760,7 +760,7 @@ func HandleListUsers(cc *hotline.ClientConn, t *hotline.Transaction) (res []hotl
 	for _, acc := range cc.Server.AccountManager.List() {
 		b, err := io.ReadAll(&acc)
 		if err != nil {
-			cc.Logger.Error("Error reading account", "Account", acc.Login, "Err", err)
+			cc.Logger.Error("Error reading account", "account", acc.Login, "err", err)
 			continue
 		}
 
@@ -831,7 +831,7 @@ func HandleUpdateUser(cc *hotline.ClientConn, t *hotline.Transaction) (res []hot
 			cc.Logger.Info("DeleteUser", "login", login)
 
 			if err := cc.Server.AccountManager.Delete(login); err != nil {
-				cc.Logger.Error("Error deleting account", "Err", err)
+				cc.Logger.Error("Error deleting account", "err", err)
 				return cc.NewErrReply(t, ErrMsgDeleteAccount)
 			}
 
@@ -1005,7 +1005,7 @@ func HandleDeleteUser(cc *hotline.ClientConn, t *hotline.Transaction) (res []hot
 	login := t.GetField(hotline.FieldUserLogin).DecodeObfuscatedString()
 
 	if err := cc.Server.AccountManager.Delete(login); err != nil {
-		cc.Logger.Error("Error deleting account", "Err", err)
+		cc.Logger.Error("Error deleting account", "err", err)
 		return cc.NewErrReply(t, ErrMsgDeleteAccount)
 	}
 
@@ -1269,7 +1269,7 @@ func HandleDisconnectUser(cc *hotline.ClientConn, t *hotline.Transaction) (res [
 		switch options[1] {
 		case 1:
 			// send message: "You are temporarily banned on this server"
-			cc.Logger.Info("Disconnect & temporarily ban " + string(clientConn.UserName))
+			cc.Logger.Info("Disconnect & temporarily ban user", "username", string(clientConn.UserName))
 
 			res = append(res, hotline.NewTransaction(
 				hotline.TranServerMsg,
@@ -1288,7 +1288,7 @@ func HandleDisconnectUser(cc *hotline.ClientConn, t *hotline.Transaction) (res [
 			}
 		case 2:
 			// send message: "You are permanently banned on this server"
-			cc.Logger.Info("Disconnect & ban " + string(clientConn.UserName))
+			cc.Logger.Info("Disconnect & ban user", "username", string(clientConn.UserName))
 
 			res = append(res, hotline.NewTransaction(
 				hotline.TranServerMsg,