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/fireNotification.dart

@@ -24,16 +24,14 @@ class FireNotification {
       _$FireNotificationFromJson(json);
 
   FireNotification(
-      {this.id,
+      {required this.id,
       required this.lat,
       required this.lon,
       required this.description,
       required this.when,
       required this.read,
       required this.sealed,
-      required this.subsId}) {
-    
-  }
+      required this.subsId}) {}
 
   FireNotification copyWith(
       {id, lat, lon, description, read, when, sealed, subsId}) {