fix(logger): prevent nil-deref panic in migrate/setting CLI paths

The package-level logger is nil until InitLogger runs, which only happens in runWebServer. The migrate and setting subcommands log without initializing it; PR #5520 added a logger.Info on a success path in MigrationRestoreVisionFlow, so 'x-ui migrate' segfaults on installs with a VLESS inbound needing Vision-flow restoration.

Initialize logger to a usable default at package load so no code path can nil-deref it, and set up the dual backend in migrateDb so migration steps are logged like runWebServer.

Fixes #5581

internal/logger/logger.go

@@ -29,7 +29,9 @@ const (
 )
 
 var (
-	logger     *logging.Logger
+	// Initialized to a usable default so logging never nil-derefs before InitLogger
+	// runs — the "migrate" and "setting" CLI subcommands log without calling it.
+	logger     = logging.MustGetLogger("x-ui")
 	fileRotate *lumberjack.Logger // nil when file backend disabled
 
 	// logBuffer maintains recent log entries in memory for web UI retrieval;