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/firesApp.dart

@@ -34,7 +34,7 @@ class _FiresAppState extends State<FiresApp> {
   static final WidgetBuilder introWidget = (context) => new IntroPage();
   static final WidgetBuilder continueWidget = (context) => new HomePage();
 
-  final Map routes = <String, WidgetBuilder>{
+  final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
     IntroPage.routeName: introWidget,
     HomePage.routeName: continueWidget,
     PrivacyPage.routeName: (BuildContext context) => new PrivacyPage(context),