fix: replace fmt.Sprintf in logger calls with structured fields and add LoopDelay default

fmt.Sprintf inside zap logger calls defeats structured logging,
making log aggregation and filtering harder. All 6 sites now use
proper zap fields (zap.Uint32, zap.Uint8, zap.String).

LoopDelay had no viper.SetDefault, so omitting it from config.json
caused a zero-value (0 ms) busy-loop in the recv loop. Default is
now 50 ms, matching config.example.json.

config/config.go

@@ -328,6 +328,7 @@ func LoadConfig() (*Config, error) {
 		Enabled:   true,
 		OutputDir: "save-backups",
 	})
+	viper.SetDefault("LoopDelay", 50)
 
 	err := viper.ReadInConfig()
 	if err != nil {