Fix remaining compilation errors: null-safety, removed packages, and nullable handling

- Fix AppState: remove removed connectivity package, fix imports (latlong2), make nullable fields optional
- Fix User model: use const empty strings in const constructor instead of null
- Fix model builders: add required ObjectId parameters to YourLocation and FireNotification
- Implement Comparable instead of extending it for BasicLocation
- Fix nullable callback handling in appActions and middleware (Completer<Null>?)
- Add null checks for BuildContext in dialogs (fireAlert, homePage, etc.)
- Fix nullable scaffold state access using ?. operator (activeFires, fireNotificationList)
- Handle nullable FireNotification and YourLocation in genericMapBottom
- Fix list type casting for widgets that can be null (<Widget?> with filtering)
- Add ObjectId imports where needed for model creation
- Fix Key? nullable parameter in introPage
- Add required parameters to markdownPage and slider constructors
- Remove connectivity-related code and imports (package removed)
- Handle nullable Completer in fetchDataMiddleware

All 104 errors resolved. 0 errors, 61 warnings/info remaining.

lib/models/basicLocation.dart

@@ -1,14 +1,11 @@
-
-class BasicLocation extends Comparable<BasicLocation> {
+class BasicLocation implements Comparable<BasicLocation> {
   final double lat;
   final double lon;
-  String description;
+  final String? description;
 
 //  static BasicLocation noLocation = new BasicLocation(lat: 0.0, lon: 0.0);
 
-  BasicLocation({required this.lat, required this.lon, this.description}) {
-    
-  }
+  BasicLocation({required this.lat, required this.lon, this.description}) {}
 
   int compareTo(BasicLocation other) {
     return lat == other.lat && lon == other.lon ? 1 : 0;