Preferences: Add API validation and update documentation (#116045)

pkg/services/preference/timezone.go

@@ -0,0 +1,21 @@
+package pref
+
+import (
+	"time"
+)
+
+// IsValidTimezone checks if the timezone string is valid.
+// It accepts:
+// - "" - uses default
+// - "utc"
+// - "browser"
+// - Any valid IANA timezone (e.g., "America/New_York", "Europe/London")
+func IsValidTimezone(timezone string) bool {
+	if timezone == "" || timezone == "utc" || timezone == "browser" {
+		return true
+	}
+
+	// try to load as IANA timezone
+	_, err := time.LoadLocation(timezone)
+	return err == nil
+}