refactor: extract logFatal helper for config error handling

Replace 14 repeated fmt.Fprintln(os.Stderr, "FATAL:", ...)/os.Exit(1)
patterns with a single logFatal function. This reduces duplication
and makes all fatal config paths testable via SetLogFatal.

Signed-off-by: Deluan <deluan@navidrome.org>

conf/export_test.go

@@ -14,8 +14,8 @@ var NormalizeSearchBackend = normalizeSearchBackend
 
 var ToPascalCase = toPascalCase
 
-func SetFatalFunc(f func(string)) func() {
-	old := fatalFunc
-	fatalFunc = f
-	return func() { fatalFunc = old }
+func SetLogFatal(f func(...any)) func() {
+	old := logFatal
+	logFatal = f
+	return func() { logFatal = old }
 }