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/user.dart

@@ -8,13 +8,13 @@ class User {
   final String token;
 
   const User.initial()
-      : this.userId = null,
-        lang = null,
-        token = null;
+      : userId = '',
+        lang = '',
+        token = '';
 
-  User({this.userId, this.lang, this.token});
+  const User({required this.userId, required this.lang, required this.token});
 
-  User copyWith({String userId, String lang, String token}) {
+  User copyWith({String? userId, String? lang, String? token}) {
     return new User(
         userId: userId ?? this.userId,
         token: token ?? this.token,
@@ -25,5 +25,4 @@ class User {
   String toString() {
     return 'User {userId: $userId, lang: $lang, token: ${ellipse(token)}';
   }
-
 }