localization: update GPS settings messages and improve handling of custom variables

lib/l10n/app_en.arb

@@ -83,14 +83,13 @@
   "settings_radioSettingsUpdated": "Radio settings updated",
   "settings_location": "Location",
   "settings_locationSubtitle": "GPS coordinates",
-  "settings_locationUpdated": "Location updated",
+  "settings_locationUpdated": "Location and GPS settings updated",
   "settings_locationBothRequired": "Enter both latitude and longitude.",
   "settings_locationInvalid": "Invalid latitude or longitude.",
   "settings_locationGPSEnable": "GPS Enable",
   "settings_locationGPSEnableSubtitle": "Enables GPS to automatically update location.",
   "settings_locationIntervalSec": "Interval for GPS (Seconds)",
   "settings_locationIntervalInvalid": "Interval must be at least 60 seconds, and less than 86400 seconds.",
-  "settings_locationUpdated": "GPS settings updated.",
   "settings_latitude": "Latitude",
   "settings_longitude": "Longitude",
   "settings_privacyMode": "Privacy Mode",

lib/screens/settings_screen.dart

@@ -433,16 +433,17 @@ class _SettingsScreenState extends State<SettingsScreen> {
     final latController = TextEditingController();
     final lonController = TextEditingController();
     final intervalController = TextEditingController();
-    intervalController.text = "900";
     latController.text = connector.selfLatitude?.toStringAsFixed(6) ?? '';
     lonController.text = connector.selfLongitude?.toStringAsFixed(6) ?? '';
-    bool hasGPS = connector.currentCustomVars!.isNotEmpty
-        ? connector.currentCustomVars!.containsKey("gps")
-        : false;
 
-    bool isGPSEnabled = hasGPS
-        ? connector.currentCustomVars!["gps"] == "1"
-        : false;
+    // Safe access to custom vars - may be null before device responds
+    final customVars = connector.currentCustomVars ?? {};
+    final bool hasGPS = customVars.containsKey("gps");
+    bool isGPSEnabled = customVars["gps"] == "1";
+
+    // Read current interval or default to 900 (15 minutes)
+    final currentInterval = int.tryParse(customVars["gps_interval"] ?? "") ?? 900;
+    intervalController.text = currentInterval.toString();
 
     showDialog(
       context: context,
@@ -521,7 +522,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
                   }
 
                   final interval = int.tryParse(intervalText);
-                  if (interval == null || interval < 60) {
+                  if (interval == null || interval < 60 || interval >= 86400) {
                     if (!context.mounted) return;
                     ScaffoldMessenger.of(context).showSnackBar(
                       SnackBar(