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/markdownPage.dart

@@ -12,7 +12,7 @@ abstract class MarkdownPage extends StatefulWidget {
   final Future<String> file;
   final String route;
 
-  MarkdownPage({this.title, this.route, this.file});
+  MarkdownPage({required this.title, required this.route, required this.file});
 
   @override
   _MarkdownPageState createState() =>
@@ -25,7 +25,8 @@ class _MarkdownPageState extends State<MarkdownPage> {
   final Future<String> file;
   String pageData = "";
 
-  _MarkdownPageState({this.title, this.file, this.route});
+  _MarkdownPageState(
+      {required this.title, required this.file, required this.route});
 
   @override
   Widget build(BuildContext context) {