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.

3 files changed, 20 insertions, 13 deletions

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,