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/redux/fetchDataMiddleware.dart

@@ -165,7 +165,7 @@ void fetchDataMiddleware(Store<AppState> store, action, NextDispatcher next) {
           });
           locSubs.subscribed = true;
         });
-      
+
         store.dispatch(new FetchYourLocationsSucceededAction(localLocations));
         persistYourLocations(localLocations);
 
@@ -182,9 +182,9 @@ void fetchDataMiddleware(Store<AppState> store, action, NextDispatcher next) {
           });
         });
 
-        Completer<Null> completer = action.refreshCallback;
-        completer.complete(null);
-            });
+        final Completer<Null>? completer = action.refreshCallback;
+        completer?.complete(null);
+      });
     }).catchError((onError) {
       // If it fails, dispatch a failure action. The reducer will
       // update the state with the error.