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/yourLocation.dart

@@ -13,22 +13,28 @@ class YourLocation {
   String description;
   bool subscribed;
   int distance;
-  int currentNumFires;
+  late int currentNumFires;
 
   factory YourLocation.fromJson(Map<String, dynamic> json) =>
       _$YourLocationFromJson(json);
 
-  static YourLocation noLocation = new YourLocation(lat: 0.0, lon: 0.0);
-  static const int withoutStats = null;
+  static late final YourLocation noLocation;
+  static const int? withoutStats = null;
+
+  static void _initNoLocation() {
+    noLocation = new YourLocation(
+        id: ObjectId(), lat: 0.0, lon: 0.0, description: '', subscribed: false);
+  }
+
   YourLocation(
-      {this.id,
+      {required this.id,
       required this.lat,
       required this.lon,
-      this.description,
+      this.description = '',
       this.distance = 10,
-      int currentNumFires = withoutStats,
-      this.subscribed =false}) {
-    
+      int? currentNumFires,
+      this.subscribed = false}) {
+    this.currentNumFires = currentNumFires ?? 0;
   }
   Map<String, dynamic> toJson() => _$YourLocationToJson(this);